If this waste is applied as a fertiliser, the water surplus increases the elution of this nutrient into the bottom soil in pervious soils. In impervious soils and in less pervious soils the imbalance between water and air in the soil is deteriorated with all adverse consequences: aerobiosis restriction, reduction in the count of soil microorganisms, denitrification and escape of valuable nitrogen in the form of N2 or N-oxides into the atmosphere. Soil acidification takes place because organic substances are not mineralised under soil anaerobiosis and they putrefy at the simultaneous production of lower fatty acids. These soil processes result in a decrease in soil productivity. Currently, its probability is increasingly higher for these reasons:

1. As a consequence of global acidification the frequency of abundant precipitation is higher in Europe throughout the year.

2. As a result of rising prices of fuels, depreciation on farm machinery and human labour force farmers apply digestates or fugates in the closest proximity of a biogas plant. It causes the overirrigation of fertilised fields even though the supplied rate of nitrogen does not deviate from the required average.

The problem of an excessively high irrigation amount has generally been known since long: it occurred in Berlin and Wroclaw irrigation fields after irrigation with municipal waste water in the 19th and 20th century, in the former socialist countries after the application of agricultural and industrial waste waters and of slurry from litterless operations of animal production. Even though nobody surely casts doubt on the fertilising value of pig slurry or starch-factory effluents, total devastation of irrigated fields and almost complete loss of their potential soil productivity were quite normal phenomena (Stehlik 1988).

Carbon dioxide is more soluble in water than methane. This phenomenon is employed to remove CO2 from biogas in water scrubbing technologies. Biogas is fed to a column where it is "washed" with counter-current water that is sprayed from the top of the column. The column is normally filled with some material to enhance the interface area promoting CO2 absorption. The CO2 is dissolved in the water that is then pumped to a "regeneration column" where CO2 is released. The regeneration of the water scrubbing process can be carried out at higher temperatures or at lower pressures. In this technology, H2S is removed with CO2. Also the purified CH4 stream (with purity up to 98%) should be dried after leaving the scrubber.

The solubility of CO2 in water strongly increases at lower temperatures. In order to reduce pumping energy, the water should be available at low temperatures. In fact, this technique is been employed in several countries with cold weather (Sweden, Switzerland, Germany, Austria, etc). Cooling down water may still be efficient for large facilities, but not for small applications.

Nowadays, water scrubbing is the most employed technique for upgrading biogas. Plants with processing capabilities from 80 to 10000 m3/hour are in operation. Main technology developers in this area are: Malmberg (www. malmberg. se), Flotech Inc. (www. flotech. com), Rosroca (www. rosroca. de), DMT (www. dirkse-milieutechniek. com), etc. In some of these webpages there is a video which actually explain graphically in detail how the process works.

The direct usage of crude or filtered vegetable oils for diesel engine fuel is possible by blending them with conventional diesel fuels in a suitable ratio. These blends are easy to obtain and keep stable for short-term use. But vegetable oils present high viscosity, acid contamination and free fatty acids that lead to gum formation by oxidation, polymerization and carbon deposition (Ranganathan, 2008). Thus, the long-term utilization of vegetable oils for fuel leads to filter clogging, nozzle blockage and deposits in the combustion chamber (Sidibe et al., 2010). Alongside the long-term problems in injection systems, filters and combustion chamber, doubts about the sustainability of using crude vegetable oil for fuels have to be considered. Vegetable oils are expensive, and their direct use in engines or as feedstock to produce petro-diesel substitutes would encounter the same economic and environmental problems that affect the conventional biodiesel and bioethanol industries (UNCTAD, 2010).

A more interesting solution is the usage of waste cooking oil (WCO; also called waste frying oil, WFO). Waste cooking oil is widely produced, inedible, and could serve as a low-cost and almost ready-to-use substitute for fossil origin diesel. As crude vegetable oil, waste cooking oil has a high viscosity. Besides, it is enriched with free fatty acids and, hence, can generate clogging problems in unmodified diesel vehicles, especially in temperate climates and during the ignition of the engine. Viscosity problems are usually bypassed by blending WCO with petrol diesel or by using transesterification to produce biodiesel (Pugazhvadivu et al., 2005; Al-Zuhair et al., 2009; Chen et al., 2009). Al-Zuhair et al. studied the production of biodiesel with lipases from Candida antarctica and Burkholderia cepacia, both free and immobilized in ceramic beads, with or without solvents. They found that clay micro­environments protected immobilized B. cepacia lipase from methanol damage (Al-Zuhair et al., 2009). Also, Pugazhvadivu et al. proposed solving the injection and filter-clogging problems by preheating the waste cooking oil (Pugazhvadivu et al., 2005), by comparing the performance of a diesel engine when using conventional diesel and waste frying oil, preheated at different temperatures, as fuel. They found that preheating the waste frying oil to 135°C improved the overall yield of the engine. In particular, the brake specific energy consumption and brake thermal efficiency were improved, and the engine exhaust CO and smoke density were reduced considerably compared to WFO preheated at 75°C. They concluded that WFO could be used as a diesel fuel by preheating it to 135°C.

The use of acid hydrolysis for the conversion of cellulose to glucose is a process that has been studied for the last 100 years. Dilute acid (0.5-1.0% sulfuric acid) pretreatment at moderate temperatures (140-190°C) can effectively remove and recover most of the hemicellulose as dissolved sugars. Furthermore lignin is disrupted and partially dissolved, increasing cellulose susceptibility to enzymes (Yang and Wyman, 2004). Under this method, glucose yields from cellulose increase with hemicellulose removal to almost 100% (Knappert et al., 1981). Dilute acid hydrolysis consists of two chemical reactions. One reaction converts cellulosic materials to sugar and the other converts sugars into other chemicals, many of which inhibit the growth of downstream fermentation microbes. The same conditions that cause the first reaction to occur simultaneously cause over-degradation of sugars and lignin, creating inhibitory compounds such as organic acids, furans, and phenols.

Partial cellulose may be degraded as oligomers or monomers during the acid pretreatment process. Sugar (glucose and xylose) yields were often reported for the pretreatment and enzyme hydrolysis stage separately, and as the total for both stages. Lloyd and Wyman (2005) reported that up to 92% of the total sugars originally available in corn stover could be recovered via coupled dilute acid pretreatment and enzymatic hydrolysis. Conditions achieving maximum individual sugar yields were often not the same as those that maximized the total sugar yields, demonstrating the importance of clearly defining pretreatment goals when optimizing the process.

Dilute-sulfuric acid pretreatment of cattails was studied using a Dionex accelerated solvent extractor (ASE) at varying acid concentrations of 0.1 to 1%, treatment temperatures of 140 to 180 °C, and residence times of 5 to 10 min. The yield of extractable products obtained from the pretreatment process increased as the final temperature, treatment time, or acid concentration increased. The highest glucose yield from the pretreatment was 55.4% of the cellulose at 180°C for 15 min with 1% sulfuric acid. The highest glucose yield from the enzyme hydrolysis stage (82.2% of the cellulose) and the highest total glucose yield for both the pretreatment and enzyme hydrolysis stages (97.1% of the cellulose) were reached at a temperature of 180°C, a sulfuric acid concentration of 0.5%, and a time of 5 min.

When switchgrass was pretreated for 60 min with 1.5% acid, the highest glucan conversion yield of 91.8% was obtained (Yang et al. 2009).

The perturbation in atmospheric trace gases (e. g., SO2, O3, CO, CO2, CH4, NO2, and CFCs, among others) is an important factor affecting climate change (Hopkin, 2007). In turn, climate change may promote changes in agricultural conditions that could have deleterious socioeconomic effects (Howden et al., 2007). Atmospheric trace gases have strong absorption bands in the infrared (IR) and interact with IR radiation emitted both by the earth’s surface and the atmosphere. This directly influences the thermal structure of the atmospheric environment and contributes to the greenhouse effect. Gases such as NH3, SO2 and their derivatives have lifetimes of only a few days, but they can have significant effects on the atmosphere (Begum, 2005). Emissions of N2O and CH4 are currently the dominant contributors. Although CO2 is the main greenhouse gas in terms of volume, others must also be considered. In agricultural practices, the main culprit is nitrous oxide (N2O), significant quantities of which are released from cultivated fields (particularly with the intensive use of fertilizers) (Snyder et al., 2009; Ceschia et al., 2010; Mander et al., 2010). Because N2O is >300 times more potent as a GHG than is CO2, even modest volumes can have significant impacts on the overall balance (Cherubini, 2010).

Harnessing the carbon sequestration capabilities of the terrestrial biosphere has been recognized as a potentially powerful, yet relatively low-cost, tool to offset carbon emissions (Dorian et al., 2006) and models for that purpose have been investigated (Werner et al., 2010). However, terrestrial carbon sequestration has been considered insufficient for meeting more than 25% of the CO2 emissions reductions that are globally required by 2050. Given that carbon sinks are the best currently available scenario, an emissions credit system has been established to provide CO2 emitters (companies or countries) with a means to satisfy the carbon liability associated with their release of carbon into the atmosphere. The emitter temporarily satisfies his liability by "storing" (for a fee) the equivalent carbon in a terrestrial carbon sink (such as a forest) (Sedjo & Marland, 2003). This concept is the application of the "willing-to-pay" principle within the international economic market. More simply, the right to emit CO2 (in the form of a carbon credit) is compensated for by growing biomass that will sequester an equivalent amount of carbon. The marketing of carbon credits has been organized to allow for rewarding activities that result in the "permanent" immobilization of CO2 in a nongaseous form. Ultimately, a carbon fee has been proposed that would be paid by industrial countries (in proportion to their emission contributions to GHG) to developing countries; these countries could then invest them in carbon mitigation practices (such as establishing or maintaining forest sinks) (Jones, 2010). The Kyoto Protocol is now legitimating activities such as revegetation, forest management, cropland management, grazing-land management, and also carbon sequestration in deep crustal layers (such as oil fields and deep saline aquifers) for trading with carbon credits (United Nations Framework Convention on Climate Change [UNFCCC], 2002). Principles of justice in proposals and policy approaches to avoided deforestation are also being pursued (Okereke & Dooley, 2010) through negotiations on Reducing Emissions from Deforestation and forest Degradation (REDD).

It has been estimated that the biological sink may attain a cumulative CO2 sequestration of 100 Gt over the next 50-100 years, with most of it in forest systems. This implies the capture of 10-20% from the anticipated net fossil fuel emissions until 2050 (Sedjo & Marland, 2003). However, carbon sequestered in the terrestrial biosphere may lack permanence. Forests may be harvested for timber that can be used to produce short-lived products or may be cleared for other purposes. Wild fires can release large amounts of sequestered carbon. Farmers may return to agricultural practices that release carbon that was previously captured. In that sense, terrestrial carbon sequestration may simply represent a delay in the flow of fossil fuel carbon to the atmosphere. However, economic incentives for carbon sequestration should increase permanent sequestration. That is, wherever and whenever there are incentives (payments) for carbon-sequestration services, one would expect more sequestration to occur than if no payments were made (Johnston & Holloway, 2007; Tollefson, 2008).

Carbon sequestration in living forests can be performed on lands with low productivity that are not suitable for agriculture or for intensive forestry and that are compatible with goals of biodiversity conservation over large areas. In contrast, to be economically viable, intensive crops for biofuels generally need land that is more productive. Intensive biofuel crops may compete with food production or even with the less-productive lands that are currently sheltering most of the earth’s biodiversity (Huston & Marland, 2003; Miles & Kapos, 2008). For example, this phenomena has been observed in Brazil, Indonesia and Malaysia, where cattle, soybean, sugarcane and palm oil may compete with standing forest (Darussalam, 2007; Laurance, 2007; Malhi et al., 2008; Stone, 2007; Venter et al., 2008). In Indonesia, this competition has disastrous consequences for wildlife. To resolve this problem, the Kyoto protocol and subsequent versions should include "wildlife credits" (in addition to carbon credits) to sustain wildlife and its buffering effect on human activities (Lovelock & Margulis, 1974). This strategy would have the advantage of recognizing the fundamental roles played by ecosystem services and to begin to account for them (Maler et al., 2008). New financial incentives are needed to act as a countervailing force to the economic pressures for deforestation (Jones, 2010). The recent agreement known as the "Bali Roadmap", which aims to extend the Kyoto Protocol beyond 2012, includes directives for providing compensation to rainforest-holding nations in exchange for control of deforestation and environment degradation. Such compensation could be managed either through international carbon markets or through voluntary funds. These directives have the potential to shift the balance of underlying economic market forces that currently favor deforestation by raising billions of dollars to pay for the ecosystem services provided by rainforests. However, to be effective they will require exceptional planning, execution and long-term follow-through. The new proposal also aims to reduce EU CO2 emissions by 30% by 2020 if a global climate deal is reached in international negotiations (if not, the cut will be 20%) (Schiermeier, 2008). The EU is also planning to protect its economy against carbon "dumpers" by applying leverage that aims to force companies that import goods from polluting countries to buy emissions permits (Barnet, 2008).

Typically, carbon-credit compensation funds are used in developing countries for establishing new long-term plantations (such as rubber trees or oil palm). One difficulty is that the actual goal of carbon sequestration can be negated in cases where the renter first illegally burns the original forest, earns the carbon-credit funds and subsequently establishes a new plantation that will never be as productive, in terms of carbon sequestration, as the original forest. In some regions, environmental crimes are not easily detected and may also not be "significantly" punished. Key recommendations to ensure the environmental sustainability of biofuels through certification (including international approaches and global monitoring) should help to control the process (Scarlat & Dallemand, 2010). Despite these problems, the carbon-credit market was stabilized as of 2007 and is not expected to collapse any further (Haag, 2007). The next few years represent a unique opportunity (perhaps the last) to maintain the resilience of biodiversity and ecosystem services (Malhi et al., 2008; Garcia-Montero et al., 2010; Hagerman et al., 2010).

Grau Baquero, Bernat Esteban, Jordi-Roger Riba, Rita Puig and Antoni Rius

Escola d’Enginyeria d’lgualada, Universitat Politecnica de Catalunya

Spain

1. Introduction

The current dependence on oil in most industrial sectors and mainly in the transport sector is unsustainable neither in short nor in long term. This encourages to consider alternatives in most industrial sectors and incentivises to promote renewable energy use. In addition, the EU is promoting or even forcing the use of renewable energies in order to accomplish the commitments under the Kyoto Protocol.

In Europe the most common biofuels in transport are biodiesel and bioethanol. These biofuels are mostly obtained from large-scale plants and its production involves serious environmental and social problems as shown by several authors (Russi, 2008; Galan et al., 2009). In this scenario it is necessary to implement other biofuels currently not present in the Spanish market.

Straight vegetable oil (SVO) is a biofuel that can be small-scale produced from rapeseed planted in dry Mediterranean areas. The small-scale production presents several advantages and is more sustainable than large-scale production as cited by several authors (Baquero et al., 2010).

This chapter presents a method to produce rapeseed and process it to obtain rapeseed oil and rapeseed cake meal from a small-scale point of view. It also shows how rapeseed oil can be used as fuel in diesel engines for agriculture self-consumption. A production, processing and use-as-fuel model for rapeseed oil is also presented, analysing environmentally and economically the use of rapeseed oil as fuel compared to other agricultural production alternatives. The results are evaluated for dry Mediterranean area conditions.

Depending on the kind of catalyst used and the selected operation conditions in the biodiesel production plant, the alcohol to oil molar ratio will present a wide variation. Adding excess alcohol is a common practice, and could serve as reference. However, excess alcohol use implies higher reactant associated costs, especially when the alcohol of choice is ethanol, which is more expensive than methanol. Thus, a more detailed approach to the system optimization in terms of minimal alcohol consumption is needed. Besides, a fine adjustment of the alcohol to oil ratio allows the maximal biodiesel production in the shortest possible time span and with the lowest energy input (Shieh et al., 2003).

The optimization is a relatively simple task when homogeneous catalysts such as sulphuric acid or sodium hydroxide are used to perform the conventional transesterification of vegetable oils with methanol. High yields are achieved with a methanol to oil ratio of 1:1 with an alkaline catalyst (although to improve the yield this proportion rises to 6:1) and a 30:1 ratio when an acid catalyst is used (Zhang et al., 2003).

However, in the case of lipase-catalyzed biodiesel, the situation is more complex and the molar ratio of alcohol to oil varies depending on the type of lipase, the use of an immobilized or free enzyme, and the alcohol used. Similar to the chemical catalysts, an increase of the molar alcohol:oil ratio elevates the efficiency of the reaction, but an excessive alcohol content inhibits and even damages the enzyme, especially when using methanol and free enzymes. Although the lipase-based solvent-free systems are under intensive research, owing to advantages such as the direct saving in solvents and the indirect cost reductions in downstream processes, the utilization of lipases does not necessary mean abandoning the use of a certain amount of solvents. The addition of solvents like f-butanol, diesel oil, hexane or dioxane to the precursors of biodiesel usually allows a better mixing of the reactants. Thus, solvents relieve the problems associated with the different water solubility of lipids and alcohols. In addition, solvents provide a more durable interaction between the enzyme and its substrates, and can favour the circulation of reactants through resins and support pores in immobilized enzyme systems. This improved circulation confers some protection to the lipases against inhibition by substrates and damages by excessive alcohols. However, solvents’ addition has to be carefully studied, since an excess of solvent or an inadequate amount of solvent can affect the enzyme activity and stability. For example, Shieh et al. studied the optimal operation conditions to transesterificate soybean oil with methanol by Rhizomucor miehei lipase immobilized on macroporous weak anionic resin beads. They found that the best transesterification rate was obtained when the methanol:oil molar proportion was 3.4:1 at 36.5°C (Shieh et al., 2003). Raita et al. studied the transesterification of palm oil with ethanol by Thermomyces lanuginosa lipase-coated microcrystals in the presence of f-butanol. In this case, the optimal conditions were ethanol to fatty acids 4:1 molar ratio and f-butanol:tryacylclycerides 1:1 molar ratio, at 45°C (Raita et al., 2010. However, Tongboriboon et al. worked on the solvent-free transesterification of used palm oil with Thermomyces lanuginosa and Candida anfarcfica lipases immobilized in porous polypropylene powder, reporting that the best yield was achieved at an ethanol to oil ratio of 3:1, and the yield decreased when the molar ratio was increased to 4:1 at 45°C (Tongboriboon et al., 2010). These authors pointed to the inhibition of the enzymes by an excessive amount of ethanol, although it is worth emphasizing that they worked on a solvent-free system, so the enzyme was relatively vulnerable to alcohol-driven damage. On the other hand, Shah et al used 4:1 ethanol to oil molar ratio as standard reaction settings in their study about the transesterification of jatropha oil with ethanol at 40°C. The experimental design consisted of a solvent-free system and three different lipases (free and immobilized on Celite), namely Chromobacferium viscosum, Candida rugosa and Porcine pancreas lipases, although they did not try different alcohol to oil molar ratios (Shah et al., 2004).

Polar solvents such as methanol, ethanol, and furfural have been used for many years to homogenize and to reduce viscosity of biomass oils (Radlein et al., 1996; Diebold and Czernik, 1997; Oasmma, 2004; Boucher et al., 2000). The immediate effects of adding these polar solvents are decreased viscosity and increased heating value. The increase in heating value for bio-oils mixed with solvents occurs because the solvent has a higher heating value than that of most bio-oils. The solvent addition reduces the oil viscosity due to the following three mechanisms: (1) physical dilution without affecting the chemical reaction rates; (2) reducing the reaction rate by molecular dilution or by changing the oil microstructure; (3) chemical reactions between the solvent and the oil components that prevent further chain growth (Oasmaa and Czernik, 1999).

Most studies have directly added solvents after pyrolysis, which works well to decrease the viscosity and increase stability and heating value. However, several recent studies showed that reacting the oil with alcohol (e. g., ethanol) and acid catalysts (e. g., acetic acid) at mild conditions by using reactive distillation, resulted in a better bio-oil quality (Mahfud et al., 2007; Xu et al., 2008; Tang et al., 2008; Oasmma, et al., 2004; Xu and Etcheverry, 2008). This process is referred to as catalytic etherification or etherification treatment in the literature (Xiong et al., 2009; Wang et al., 2010; Hilten et al., 2010; Yu et al., 2009).

The chemical reactions that can occur between the bio-oil and methanol or ethanol are esterification and acetalization (Fig.6). In such a case, the reactive molecules of bio-oil like organic acids and aldehydes are converted by the reactions with alcohols to esters and acetals, respectively. Thus, in addition to the decrease in viscosity and in the aging rate, they also lead to other desirable changes, such as reduced acidity, improved volatility and heating value, and better miscibility with diesel fuels.

Most environmental catalysts applied in bio-oil upgrading are heterogeneous catalysts. Solid acid catalysts, solid base catalysts (Zhang et al., 2006), ionic liquid catalysts (Xiong et al., 2009), HZSM-5, and aluminum silicate catalysts were investigated for esterification of bio-oils (Peng et al., 2008, 2009).

Considering the simplicity, the low cost of some solvents such as methanol and their beneficial effects on the oil, this method seems to be the most practical approach for bio-oil quality upgrading.

The main components of plant oils are the fatty acids and their derivatives the mono-, di — and triacylglycerides. Tri-acyl glycerides make up 95% of plant oils. Glycerides are esters formed by fatty acid condensation with tri-alcohol glycerol (propanetriol). Depending on the number of fatty acids fixed on the glycerol molecule, one can have mono-, di — or triacylglycerides. Of course, the fatty acids can be the same or different. As stated in the introduction, biodiesel can be obtained by esterification or transesterification. Esterification is the process by which a fatty acid reacts with a mono-alcohol to form an ester. The esterification reaction is catalyzed by acids. Esterification is commonly used as a step in the process of biodiesel fabrication to eliminate FFAs from low-quality oil with high acid content. Transesterification (or alcoholysis) is the displacement of alcohol from an ester by another alcohol in a process similar to hydrolysis. This process has been widely used to reduce triglyceride viscosity. The transesterification reaction is represented by the general equation (5).

RCOOR’ + R"OH ^ RCOOR" + R’OH (5)

This stepwise reaction occurs through the successive formation of di — and monoglycerides as intermediate products (Canakci et al., 2006). Theoretically, transesterification requires three alcohol molecules for one triglyceride molecule; however, an excess of alcohol is necessary because the three intermediate reactions are reversible (Marchetti et al., 2007; Om

Tapanes et al., 2008). After the reaction period, the glycerol-rich phase is separated from the ester layer by decantation or centrifugation. The resulting ester phase (crude biodiesel) contains contaminants such as methanol, glycerides, soaps, catalysts, or glycerol that must be purified to comply with the European Standard EN 14214.

Different technologies can be used for biodiesel production; these include chemical or enzyme catalysis and supercritical alcohol treatment (Demirbas, 2008b). EN 14214 establishes 25 parameters that must be assessed to certify the biodiesel quality.

In conventional transesterification and esterification processes for the production of biodiesel, strong alkalis or acids are used as chemical catalysts. These processes are highly energy consumptive and the poor reaction selectivity that often results from the physicochemical synthesis justifies the ongoing research on enzymatic catalysis. In addition, an extra purification step is required to remove glycerol, water, and other contaminants from alkyl-esters.

The base catalysis is much faster than the acid catalysis. Low cost and favorable kinetics have turned NaOH into the most-used catalyst in the industry. However, soap and emulsion can be formed during the reaction and complicate the purification process.

Crucial challenges to the oil industry are evolving, as the demand for energy services (mobility, lighting, rotary movement, heating and cooling) increases, which with the current technology setup, is translated into expanding demand for liquid fossil fuels. Oil producers and refiners face difficulties finding sufficient good quality crude oil in adequate amounts and reasonable costs to meet growing demand over the long run, while users and the public at large are pushing for environmental improvements, such as better air quality in the immediate future. Moreover, concerns about the impacts of climate change caused by increased greenhouse gases emissions from the production and use of liquid hydrocarbons may eventually force a transition to climate friendly energy services providing systems. This offers biofuels a market penetration opportunity in the transition towards a yet undefined new energy future.

Under this background, the oil refining industry in the USA and the European Union has been stagnant. It has been immobilized by environmental obstacles posed by an articulated public, augmented by a "not in my backyard" attitude that makes it difficult to build new refineries. In addition, declining margins for refined products may have led major players to focus more on the upstream.

On the other hand, refining is expanding in other parts of the world, such as India, China, Brazil and the Middle East, as these countries develop and the oil producers attempt to add more value to their resources. An evidence of the shift of refining towards developing economies is the fact that the largest refinery in the world is in India and belongs to Reliance. But, all over the world, the increase in the long-term marginal cost of oil combined with environmental pressures and stricter government regulations and mandates are likely to lead to the decline of the centrality of oil in the global energy mix in favor of natural gas. This shift in dominance happened to wood and coal over the past two centuries and is now happening to oil. Oil companies are increasingly calling themselves energy companies. Some of them will leverage their current oil production margins to make a smooth transition to alternatives over time. A profitable transition to alternatives in the oil economy would require a gradual transfer of oil profits into green investments and the stretching of current oil supplies.