Rapeseed is one of the most important oilseeds in the world, ranking second in respect to production after soybean (Division, 2014). Back in 2005, the European Union (EU) was the world leading biodiesel producer and third in biofuel production; 60% from the total of 10.2 billion liters of biodiesel produced worldwide in 2007 was produced in the EU. Rapeseed, cultivated in most European countries, accounted for more than half of the European production of biodiesel with a share of 79% of all EU biodiesel feedstock crops in 2008 (van Duren et al., 2015).

Rapeseed-based biodiesel production has been widely studied in terms of optimization and kinetics of alkali catalyzed transesterification reaction (Luque et al., 2011). Recently, production of solid base catalysts, such as Ca/Zr mixed oxide catalysts (Liu et al., 2015), CaO-based catalysts or 4-sulfophenyl activated carbon-based solid acid catalyst, has been reported with a performance similar to commercial heterogeneous catalyst Amberlyst-15 (Malins et al., 2015). Present researches are also focusing on the use of supercritical ethanol and methanol as reagents to avoid drawbacks due to the use of homogeneous catalysts (Farobie and Matsumura, 2015a,b). Technoeconomic and performance studies on the use of supercritical methanol concluded that lower direct costs and environmental impacts are achieved at highest biodiesel yields, where oil consumption per unit of biodiesel produced is the lowest, despite a significant increase in the reaction temperature (Tomic et al., 2015). The inclusion of auxiliary energies, such as microwave heating (Azcan and Danisman, 2008), or ultrasound (Saez-Bastante et al., 2014a,b) to improve biodiesel conversion rate has also been studied. The consequences of use and production of rapeseed-based biodiesel, such as performance in diesel engines and combustion kinetics, both experimental and simulated numerically (Alviso et al., 2015), LCA related to cultivation conditions (Queiros et al., 2015), or the degradation of sealing materials in aviation (Dubovsky et al., 2015) have been of major concern recently.

Canola, the name of which derives from Canadian oil with low erucic acid, is a rapeseed cultivar (Brassica napus L. and B. campestris L.), with a content of 40% oil and a high yield of oil per acre (127–160 gallons/acre) (Pahl, 2008). The main use of the oilseeds is human consumption, due to the lower level of erucic acid compared to traditional rapeseed oils. It is also used to produce livestock feed due to reduced levels of the toxin glucosinolates in the cake.

Canola-based biodiesel gels at a lower temperature than the one produced from other feedstocks make it a more suitable fuel for colder regions, with a “cloud point” of 1°C and a “pour point” of −9°C (Peterson et al., 1997). The Canola Council of Canada published in 2010 an LCA study on canola biodiesel that shows a crop with a good energy balance and a lower GHG emissions profile when cultivated in Canada rather than in Europe. These effects are due to differences in the agronomic process: low annual precipitation (less N₂O emissions), alkaline soils (no pH adjustment required), use of ammonium-type fertilizers (with lower emissions than nitrate ones), and conservation tillage practices, among other factors (Inc., 2010).

Recent studies have shown a good performance and possibility of controlling transesterification reaction when heterogeneous catalysts based on functionalized CaO nanoparticles (Degirmenbasi et al., 2015), or honeycomb monolithic catalysts, formulated by impregnation with various metals such as ZnO, Na₂O, MgO, and CaO (Kwon et al., 2015), are used for canola biodiesel production. Enzymatic catalysis using Alcaligenes sp. lipase revealed the potential of biological and environmentally friendly catalysts to replace conventional homogeneous processes, even though they still present some inhibitory effects of methanol (Soler et al., 2016). Experiments under supercritical conditions have also been performed for canola biodiesel production. Farobie et al. (2015) proposed a spiral reactor, as effective as a conventional one, with the advantage of a better performance in terms of heat recovery, using supercritical ethanol and supercritical t-butyl methyl ether (MTBE) (Farobie and Matsumura, 2015a,b).

Finally, biolubricants based on canola biodiesel have the potential to substitute petroleum-based automotive lubricants; thus they present low cloud and pour point properties, good friction and antiwear properties, low phase transition temperature, and low viscosity (Sharma et al., 2015).

Recent studies of sunflower-based biodiesel production are focused on the sustainability of the production process (reduction of water and energy inputs, and catalyst reuse) as well as on the simplification of the operation process.

Solid catalysts based on CaO are highly basic, require mild reaction conditions for a high biodiesel yield, have low or no cost, and can be produced easily from biobased materials or wastes. In this sense, Kostic et al. reported the production of sunflower fatty acid methyl esters (FAME) using a CaO-rich palm kernel shell biochar catalyst, obtained from a gasifier for electricity production, demonstrating the potential of low-cost basic catalysts in transesterification reactions (Kostic et al., 2016). Severe calcination of eggshells provides a uniform CaCO₃/CaO-based catalyst suitable for sunflower transesterification that loses activity in the presence of atmospheric air but can be recovered by methanol washing (Reyero et al., 2015). Calcium diglyceroxide (CaDG) catalyst, synthesized by mechanochemical treatment of lime-based CaO and glycerol, was reported to acts as an emulsifier and therefore to increase the interfacial area between oil and methanol in sunflower FAME production (Lukic et al., 2016). Miladinovic et al. also demonstrated a good performance of quicklime bits-based CaO catalyst on a packed-bed reactor for the continuous production of sunflower-based FAME (Miladinovic et al., 2015).

New trends in the field of biodiesel production are oriented toward the use of ethanol instead of methanol, due to its higher oil-dissolving power, lower toxicity and biodegradability (Anastopoulos et al., 2013). Fatty acid ethyl esters (FAEEs) also present several benefits, in comparison with FAME, such as higher values for heat content and cetane number, lower cloud and pour points, lower smoke density, lower nitrogen oxide and carbon monoxide emissions, and completely biorenewable origin. Heterogeneously catalyzed ethanolysis of sunflower oil was studied by several authors using different catalysts such as CaO (Avramovic et al., 2015), calcium zincate (Miguel Rubio-Caballero et al., 2013), or calcium ethoxide Ca(OCH₂CH₃)₂ (Anastopoulos et al., 2013) for basic transesterification, as well as zirconium sulfate supported on MCM-41 silica as acid ethanolysis catalyst (Jimenez-Morales et al., 2011).

Despite research on heterogeneous catalysts having taken place for the last three decades, to date several disadvantages make them less cost competitive and not as environmentally friendly as traditionally used homogeneous catalysts. As an example, the excellence of CaO-based catalysts are numerous and well proven, but they still remain distant from the industry due to their low resistance to water and CO₂, low attrition endurance, and solubility in biodiesel and alcoholic phases, which results in ion concentrations exceeding the limits imposed by the European Norm EN14214 (Micic et al., 2015).

Auxiliary energies, like low-frequency ultrasonication using ethanol (Georgogianni et al., 2008), and methanol in combination with FTIR (Fourier transform infrared) method to monitor the reaction (Reyman et al., 2014), have been proposed to enhance the reaction yield in transesterification reactions for sunflower-based biodiesel production. Ultrasound technologies have also been researched to reduce methanol excess and enzyme dosage during biodiesel production, using immobilized lipases (eg, from Thermomyces lanuginosus), resulting in a cleaner process (Subhedar et al., 2015), and for online monitoring of the transesterification reaction means low-power ultrasound and pulse/echo techniques (Figueiredo et al., 2015). Microwave-assisted transesterification has been extensively used with basic and acid heterogeneous catalysts demonstrating that those based on calcium oxide (CaO) and potassium carbonate, pure or supported by alumina, were the most efficient when using sunflower as raw material (Dall'Oglio et al., 2014). The combination of microwaves and enzymatic catalyst for the production of FAME (Narowska et al., 2015) and FAEE (Queiroz et al., 2015) using Candida antarctica-based enzymes was reported to be faster and to provide higher yields than using conventionally heated reactors.

Due to high productivity of palm oil trees, palm oil production has increased in the last 20 years being to date the most important oil worldwide (61.44 MMT in 2014/15), with production located mainly in low-lying, wet, tropical regions, such as Indonesia (35.0 MMT), Malaysia (21.0 MMT), and Thailand (2.2 MMT) (Agriculture, 2015). Unfortunately, rainforest also occurs in these areas, and therefore about 3.5 million hectares of forest in Indonesia, Malaysia, and Papua New Guinea were replaced by palm tree intensive cultivars in the last two decades, which implied the release of large amounts of CO₂ (when peat soils are cleared and drained), and loss of clean waters and fertile soils (Nature, 2010). A good representation of a growing demanding shift to sustainable palm oil production is the 18% of global palm oil production that was certified as sustainable by the non-for-profit association Roundtable on Sustainable Palm Oil (RSPO) in 2014. Sustainability criteria required to obtain this certification are: land used may not contain significant biodiversity, wildlife habitat or other environmental values, and exploitation should meet certain environmental, social and economic standards (Oil, 2015).

Palm oil is extensively used for cooking, cosmetics, and biofuel production using homogeneous catalysts (Darnoko and Cheryan, 2000; Crabbe et al., 2001). The main research field on biodiesel production from palm oil is focused on basic and acidic heterogeneous catalysis, searching for renewable bio-based materials such as incomplete carbonized glucose and starch (Lokman et al., 2016) for supercritical transesterification. Inorganic catalysts based on nickel (Ni/HZSM-5) (Chen et al., 2016) or mixed oxide catalysts based on CaO-CeO₂ (Wong et al., 2015), among others, have also been tested, showing good results but also some active phase leaching and pore inactivation by filling.

Due to thermal and oxidation instability of FAME produced meaning transesterification, other methods such as catalytic cracking and hydrodeoxygenation of oils, to produce fuels or blending components, are the objective of the study. Even though catalytic cracking is not hydrogen-consuming, it exhibits some drawbacks such as low selectivity, side reactions of cyclization, and formation of aromatics. Wang et al. proposed a hydrogenation process for palm oil using a Ni-Mo-W/γ-Al₂O₃-ZSM-5 catalyst, yielding a biodiesel that almost conformed to the European EN-590 standard norm (Wang et al., 2015).

Concerning energy saving in the industrial process, a continuous process for the production of palm-oil-based biodiesel in a microwave reactor was demonstrated to be less time- and energy-consuming than traditional methods, while providing a 99.4% yield on biodiesel in accordance with EN/ASTM standards (Choedkiatsakul et al., 2015). Simultaneous ultrasound–microwave irradiation for a transesterification process with methanol resulted in a completed conversion within 2.2 min, yielding a 97.53% of FAME reducing temperature to 58.4°C.

Low-cost palm stearin, the solid fraction of palm oil, produced by partial crystallization under temperature-controlled conditions, is normally used for food applications but it causes manufacturing problems because of its low plasticity properties in edible fat end-products due to a high saturation degree in the fatty acids profile: 1–2% C14:0, 47–74% C16:0, and 4–6% C18:0. Theam et al. proposed the production of stearin-based biodiesel meaning heterogeneous metal doped calcium methoxide based catalyst, with promising FAME yield results, even though better conditioning of catalyst is necessary to improve its durability and performance (Theam et al., 2015).

Good performance of palm oil biodiesel and its blends in diesel engines was already reported and can be consulted in the previous edition of this book (Luque et al., 2011).

Soybean oil-based biodiesel has been produced via homogeneous catalyst in the presence of methanol for more than 20 years but its fatty acid composition needs to be genetically modified in order to produce a biodiesel viable for colder regions (Luque et al., 2011).

In recent years, research have been conducted with heterogeneous catalysts mainly based on calcium, such as CaFeAl mixed oxide (Lu et al., 2015), magnetic nanoparticle MgFe₂O₄@CaO (Liu et al., 2016) or Ca-Mg-Al hydrotalcites (Xu et al., 2015) with good stability and recyclability properties.

Use of heterogeneous and homogeneous catalysts reinforced by auxiliary energies like microwaves (Li et al., 2013; Muley and Boldor, 2013; Ye et al., 2014) or ultrasound (Yu et al., 2010) generated interesting results concerning catalyst reuse as well as time and energy savings.

Soapstock acid oil, a concentrated by-product of the soybean oil refining process based on fatty acid salts, was proposed by Soares et al. as a raw material for the production of biodiesel via acid heterogeneous catalysts using ethanol. The esterification reaction was conducted in a packed-bed bioreactor containing a lipase-rich fermented solid (sugarcane bagasse and sunflower seed meal fermented by Burkholderia cepacia) with a configuration that avoided inhibition of the catalyst by the presence of ethanol (Soares et al., 2015).

Combustion and emission characteristic tests on soybean oil biodiesel have been performed to assess the health effects associated with soybean-based biodiesel emissions. A program at the US Environmental Protection Agency (EPA) showed recently that particulate mass (PM) emissions were 30% lower with B100 combustion, compared to B0 (pure petroleum-derived diesel). Moreover, the latest results were also richer in CO, while being slightly lower in NO and organic acids than B100 (Mutlu et al., 2015).

Different engine configurations and working pressures, oxygen concentration and low-temperature combustion models have been extensively studied in order to reduce oxides of nitrogen and unburned hydrocarbons, as well as to improve the combustion efficiency of soybean-based biodiesel (Narayanan and Jacobs, 2015). Different ranges of intake pressure and oxygen concentration in a compression-ignition engine were reported by Kim et al. to have influence on thermal efficiency and CO emissions, but not on NO_x ones (Kim et al., 2014). Soot formation in biodiesel combustion represents a major concern for researchers. Xiao et al. studied the influence of temperature and oxygen concentration over soot appearance and concentration finding an opposite trend on soot creation behavior for different temperature flames: soot formation was delayed at lower flame temperatures (800 K) and decreased when lowering oxygen concentrations, while under higher temperatures (1000 K), soot mass increased while decreasing oxygen concentrations (Xiao et al., 2014). New geometries for engines, such as chamfered-bowl pistons were also proved to reduce soot, and provide a wider fuel distribution and enhanced combustion under low-temperature conditions (Kim et al., 2015).

Enhancement of fuel properties and emission levels were the main targets when studying blends of soybean-based biodiesel with n-butyl ether, that promotes the atomization of biodiesel (Guan et al., 2015), canola-oil-based biodiesel (Lee et al., 2014), alumina nanoparticles, ethanol and isopropanol (Shaafi and Velraj, 2015), or field-cress and meadow oils (Moser, 2016).

Most peanuts grown around the world are used for oil production, peanut butter, confections, and snack products (Yu et al., 2007). Even though Rudolf Diesel ran the diesel engine for the first time in 1900 using pure peanut oil (Luque et al., 2011), its biodiesel is not of major importance among researchers, most probably due to its bad cold-flow properties. Studies about peanut oil-based biodiesel have been focused on the reduction of long-chain saturated acid concentration, using different methods such as winterization (Perez et al., 2010), addition of antioxidants to prevent oleate and linoleate ester oxidation (Pinto et al., 2015), or reduction of the production costs using in-field shelling equipment (Butts et al., 2009) and heterogeneous bio-based catalysts (Shah and Gupta, 2008).

Cotton fiber grows around the seeds and is used to make natural fiber-cloth (Dorado, 2008), while the seeds contain only approximately 16.5% of oil (Bailey, 1984) which is used mainly for the production of cooking oil, margarine and nowadays, after a deodorization process, it is also used in oil dressings and mayonnaises.

The same as other vegetable oils, production of cottonseed-based biodiesel has been conducted under inorganic heterogeneous catalysts, such as ethanolysis with CaO-Mg/Al₂O_3, (Mahdavi and Monajemi, 2014) or in situ extraction and biodiesel production with magnetic S₂O₈/ZrO₂-TiO₂-Fe₃O₄ and methyl acetate (Wu et al., 2014). Pseudomonas fluorescences (Karuppasamy et al., 2013) and Rhizopus oryzae (Athalye et al., 2013) lipases were also studied as biocatalysts for FAME production with cottonseed oil.

Most interesting investigations in this field include different approaches to the biorefinery concept involving cottonseed. To serve as an example, Zhu et al. proposed the production of biodiesel, sterols, gossypol, and raffinose and nontoxic cottonseed meal in an integrated biorefinery, by a two-phase extraction process, using supercritical methanol (Zhu et al., 2014). Simultaneous production of alpha-tocopherol (a natural antioxidant) and FAME was also presented as a viable biorefinery concept (Zhu et al., 2012).

Cottonseed methyl esters were tested in a four-stroke locomotive diesel resulting in a 0.7% loss of thermal efficiency, 32% reduction of particulate matter emissions, increase of NO_x emissions as a function of several combustion parameters (eg, O/C ratio or injection timing) and a brake specific fuel consumption (BSFC) 13.4% higher than pure petrodiesel (Gautam and Agarwal, 2013).

Its higher content of linoleic and linolenic acids, compared to pure canola oil, provides green seed canola biodiesel (GSCB) with a lower cloud point and good fuel quality parameters, but its oxidation stability is lower than required by the international standards and needs to be improved to be considered a viable diesel fuel alternative (Kulkarni et al., 2006). In this sense, Issariyakul and Dalai demonstrated that biodiesel produced via homogeneous KOH-catalyst, applying a montmorillonite K10 blanch pretreatment to remove pigments from green seed canola oil, shows better oxidative stability (Issariyakul and Dalai, 2010). Baroi and Dalai discovered a solid acid catalyst (12-tungstophosphoric acid) for simultaneous esterification and transesterification of green seed canola oil, able to adsorb chlorophyll from the feedstock, improving biodiesel quality (Baroi and Dalai, 2013).

Production of GSCB by homogeneous and heterogeneous acid catalysis has been evaluated in terms of sustainability including process economics, process safety, environmental impact, and process energy efficiency. The most interesting conclusion of this study was that whenever feedstock price is under $0.35/kg, both catalysts configurations are economically profitable, but the heterogeneous acid catalyzed process is safer, creates less environmental impact, and is more energy-efficient, and therefore more sustainable (Baroi and Dalai, 2015).

The fatty acids profile of C. inophyllum, shown in Table 5.4, is mostly rich in unsaturated oleic (C18:1) and linoleic (C18:2) acids. Due to its high content of free fatty acids (FFA), and therefore high viscosity, that has removed it from biodiesel production for many years, it was selected by Muthukumaran et al. for the production of biofuel through a cracking process, using as catalyst inexpensive fly ash, improving fuel viscosity and calorific value when compared to the transesterification process (Muthukumaran et al., 2015). Blends of cracked end-product were tested in a diesel engine showing that B25 had comparable emissions and brake thermal efficiency to diesel, and that modifications on diesel engine must be accomplished to get better performance with pure biofuel.

C.I. fruit shell was also used for the production of pyrolytic oil by Alagu et al. by both thermal and catalytic (zeolite, kaolin, and Al₂O₃) pyrolysis processes, demonstrating that zeolite catalyzed pyrolysis generates a biofuel with improved calorific value and acidity (Alagu et al., 2015). C.I.-based trimethylpropane ester was also evaluated as a biodegradable lubricant in substitution of commercial lubricant and paraffin mineral oil with encouraging results (Habibullah et al., 2015a,b).

Recently, Atabani and Cesar reported the feasibility of C.I. as raw material for second-generation biodiesel production, considering its chemical properties, fatty acid composition, production technologies, and engine performance (Atabani and Cesar, 2014). Most researchers complete a minimum of two steps in the production process of CIBD (pre-esterification/transesterification) in order to avoid soap formation in the presence of FFA (Jahirul et al., 2014). Other authors also propose a previous degumming step (Ong et al., 2014). Long-chain unsaturated fatty acids esters contained in C.I.-based biodiesel are highly prone to oxidation. Synthetic antioxidant pyrogallol added at 500 ppm (Fattah et al., 2014a,b) and 2-tert-butylbenzene-1,4-diol (TBHQ) at 2000 ppm concentration (Fattah et al., 2014a,b) are good candidates to delay this degeneration stage.

Bio-based heterogeneous catalysts, such as renewable cellulose/starch-derived catalysts (Ayodele and Dawodu, 2014a,b) or immobilized Rhizopus oryzae cells (Arumugam and Ponnusami, 2014), have been reported as good candidates to improve the efficiency and sustainability of this nascent biofuel.

Engine tests carried out in recent years demonstrate good properties of this second-generation biodiesel as a lubricant in blends with traditional diesel (Habibullah et al., 2015a,b) that may be enhanced with addition (5–10%) of the oxygenated cold starting additive n-butanol (Imtenan et al., 2015), or gas to fuel (GTL, synthesized by methane reforming, Fischer–Tropsch synthesis or hydrocracking processes) in a blending mix containing 50% diesel, 30% CIBD, and 20% GTL (Sajjad et al., 2015). Comparative tests determined that the combustion duration of CIBD is higher than diesel, while the ignition delay period is shorter (Nayak et al., 2015). It is also proved that CO and HC emissions are reduced in blends with diesel, while NO_x concentration in exhaust gas is increasing with higher concentrations of CIBD in blends (Rahman et al., 2013).

Several authors have explored the potential of Annona oil for second-generation biodiesel (AOBD) production as its low acid value and fatty acid profile (rich in oleic and palmitic acids) bestows excellent properties on AOBD, meeting the international standards ASTM D6751 (Reyes-Trejo et al., 2014) and EN14214 (Branco et al., 2010). Characterization of several Annona species showed different yields and fatty acid profiles as Table 5.5 depicts. The greatest differences have been found for palmitic acid (C16:0), oleic acid (C18:1), and linoleic acid (C18:2), therefore affecting biodiesel properties from different Annona species.

Engine tests with Annona methyl esters (AME) aim to find optimal engine design parameters (eg, injection pressure and timing, compression ratio) regarding gas emissions, BTE, or specific fuel consumption (SFC), among other performance quality parameters. A B20 blend with diesel has been found to be optimal with no drawbacks or modifications on engine performance (Ramalingam et al., 2014; Senthil and Silambarasan, 2015a,b). Moreover NO_x emissions in AOBD, as well as CO, smoke and HC, may be considerably reduced, compared to neat diesel, by addition of antioxidant L-ascorbic acid (200 ppm) to AME (Senthil and Silambarasan, 2015a,b).

A biorefinery approach using Annona cherimolla Mill. seeds was presented by Branco et al. including valorization of the residual lignocellulosic fraction that remains after oil extraction. Hemicelluloses were removed from the solid fraction by autohydrolysis, generating nondigestible oligosaccharides liable to industrial processing for food, pharmacy or cosmetic applications. The remaining solids presented high enzymatic digestibility and were rich in cellulose, representing a good raw material for further valorization routes (eg, bioethanol production) (Branco et al., 2015).

It has been proposed for biodiesel production in a one-step esterification process using heterogeneous acid Si-based catalyst (Kafuku et al., 2010), with better results than a noncatalytic supercritical methanol process, that still needs to achieve higher conversion yields and high temperature stability (Kafuku et al., 2011). One-step homogeneous transesterification process (using KOH) was also studied, yielding a maximum of 89.6% FAME, with good cold flow and lubrication properties, but low oxidation stability compared to ASTM D6751 and EN14214 norms (Kafuku and Mbarawa, 2010; Kivevele and Mbarawa, 2010). Addition of antioxidants seems necessary to prevent oxidation of linoleic methyl esters (70% approx.). Synthetic antioxidants such as pyrogallol (PY), propyl gallate (PG), butylated hydroxianisole (Kivevele et al., 2011b), and several transition metals (Fe, Ni, Mn, Co, and Cu) were studied, demonstrating best performances for PY and Cu, respectively (Kivevele and Huan, 2015).

The effects of antioxidant addition on engine performance, exhaust emissions and combustion parameters were also tested showing no effect on combustion characteristics, low effect on exhaust emissions, and lower brake specific fuel consumption (BSFC) when oxidants PY and PG were added (Kivevele et al., 2011a). Blends of Croton megalocarpus oil, butanol, and diesel were also tested for engine performance and gas emissions, obtaining higher BSEC, comparable heat release rate, and lower CO₂ and smoke emissions compared to pure diesel (Lujaji et al., 2011). A 6.5-KWe electricity generator prototype, running also on pure Croton megalocarpuis oil, was constructed aiming to solve electricity supply problems in subSaharan Africa, with promising results (Wu et al., 2013).

Cold press extraction is the traditional way to obtain this nonedible oil. The need for a high-yield, high-quality and fast neem oil extraction process was the motivation of Nde et al. to investigate the use of alternative energies (eg, microwaves), demonstrating a good capacity for high oil volume extraction without significant effects on acid number and fatty acid profile of the final product (Nde et al., 2015). Fatty acid profile of neem oil (see Table 5.4) reveals a high FFA content (approx. 24.4 mg KOH/g oil). Biodiesel production implies therefore a two-stage process to avoid undesirable soap formation, which is difficult in biodiesel purification (Betiku et al., 2014). SathyaSelvabala et al. proposed a homogeneous pre-esterification process using a phosphoric-acid-based catalyst, reducing its FFA content to 1.8 mg KOH/g oil (SathyaSelvabala et al., 2010). Mathematical prediction models for the ultrasonicated production of biodiesel from neem oil were studied by Prakash and Priya, demonstrating the boundaries of artificial neural networks (ANNs) on the prediction of process performance (Maran and Priya, 2015).

The author and colleagues have already written a revision on the valorization processes for waste cooking or frying oils (WFO) (Pinzi et al., 2014). Nevertheless, they deserve a special mention in this section due to its high quantity (15 million tons per year), easy availability and low-cost transformation methods (Lopresto et al., 2015). The conversion of all available WFO into biodiesel would cover the world demand for biofuels, increasing production sustainability, eliminating a harmful waste from the environment and overcoming the competition with food markets.

The main challenges for the valorization of this waste are: optimization of the process design, selection of low-cost/high-efficiency pretreatments, and use of high-yielding, low-cost, reusable biocatalysts. Heterogeneous biocatalytic transesterification of WFO within the biorefinery concept was proposed recently by Tan et al. with promising results for waste ostrich- and chicken-eggshell CaO-based catalysts (Tan et al., 2015). As with other raw materials, WFO have also been proposed for enzymatic transesterification. To serve as an example, Lopresto et al. used Pseudomonas cepacia immobilized lipases and ethanol, added in three steps (because of lipase inhibition), for the production of WFO ethyl esters. Laboratory tests demonstrated a good performance of this biocatalyst and reflected the need for further investigation on catalyst deactivation effects (Lopresto et al., 2015). Singh and Patel proposed the use of mono lacunary phosphotungstate, anchored to MCM-41 (a recyclable catalyst) for low-cost WFO biodiesel production (Singh and Patel, 2015).

Supercritical ethanol was used together with ionic liquid [HMim][HSO₄] catalyst, yielding 97.6% biofuel in only 45 min, and the catalyst was not affected by high pressures, temperatures, or the presence of water, which implies a sustainable alternative for WFO valorization (Caldas et al., 2016).

Moreover, laboratory experiments have demonstrated that the use of ultrasound enhances the production of WFO biodiesel when using heterogeneous catalysts such as calcium diglyceroxide (Gupta et al., 2015) or sulfonated carbon (Maneechakr et al., 2015), among others.

Engine tests have been performed lately to assess the differences between methyl and ethyl esters of WFO and its blends with diesel (Sanli et al., 2015), as well as blends with butanol containing water (5%) and diesel (Tsai et al., 2015) with interesting results.

Deoxygenation of WFO for the production of biofuel and chemicals via catalytic cracking (FCC-ECAT enhanced with ZSM-5) generated a gasoline similar to that obtained for vacuum gas oil cracking, without formation of organic oxygenates such as phenolics, esters, or carboxylic acids (Lovas et al., 2015). Kinetic models for thermal cracking (fast pyrolysis) of WFO to produce hydrocarbons are able to describe the reaction pathways of different cracking products, and to group them based on the number of carbon atoms in the hydrocarbon chain: WFO (>18C), heavy bio-oil (C12–C18), light bio-oil (C4–C11), and bio-gas (<C4) (Meier et al., 2015).

Waste soybean and palm oils (by-product of the purification process of palm oil) were proposed for a single-step process to generate jet-fuel (aviation is responsible of 12% of CO₂ emissions). Results showed a degree of oxygen removal of 95%, without using hydrogen, which are promising results for the sustainability of aviation transportation (Choi et al., 2015).

The fish processing industry discharges approximately 45% of total captures in the form of heads, skin, viscera, etc. containing 1.4–40.1% (w/w) of oil (Zuta et al., 2003). Only omega-3 rich oils, produced in large quantities, are valuable for edible purposes, the rest may be valorized as raw material for biofuels. Properties of bio-oils derived from fish processing are very similar to petroleum-based fuels and depend on the extraction system. Adeoti and Hawboldt compared modified fishmeal, CO₂ supercritical extraction and soxhlet extraction methods for oil quality properties and extraction yield. The results showed that supercritical-CO₂ method extracted 91% of total oil, containing lower quantities of FFA, and therefore demonstrating better fuel properties and the possibility of using it as a heating oil to meet internal energy demand (Adeoti and Hawboldt, 2015). Leftovers of the salmon industry were studied to develop a kinetic model for the production of methyl esters in a two-step process including homogeneous pre-esterification (Serrano et al., 2015). The results corroborate that cold flow properties and oxygen stability of this kind of oils can limit its use as diesel-engine fuels, and also oil price affects the viability of this sustainable valorization process.

Waste-transformer oil (WTO) is a petroleum-based mineral oil, the disposal of which causes severe environmental problems. Its valorization was studied by Yadav et al. among others, for the production of diesel-like fuels via catalytic cracking (Yadav et al., 2015) and transesterification reaction (Yadav and Saravanan, 2015). This oil presents high viscosity that prevents its direct use in diesel engines, and was treated in a two-step process with sulfuric acid followed by a transesterification reaction in the presence of alkali catalyst and alcohol. The hydrocarbon fuel obtained was not rich in methyl esters as expected, but in cyclo-hexenol and oxabicyclo-heptane, and therefore engine modifications are required for its use. On the other hand, catalytically cracked WTO was found adequate for use in diesel engines according to ASTM standards. Blends of this hydrocarbon fuel with diesel improved greatly its BTE and PHRR, reducing emissions of HC, CO, and smoke with an increase in NO_x emissions.

Synthetic single-use plastic waste is normally placed in landfill areas (40% of global production), incinerated (150 million tons/year) which implies dangerous emissions of hydrogen cyanide, or disposed of to the sea (García, 2012). Its recycling process is very tedious and time-consuming. Moreover, the presence of additives such as pigments, coatings, or fillers limits the use of the final recycled material. Waste plastic oil (PO) can be obtained by pyrolysis and used in diesel engines mixed with petroleum-derived diesel. Nevertheless, the performance of PO25, PO50, and PO75 blends has shown that combustion characteristics are gravely affected and therefore this route should not be considered as a valorization solution for waste plastics (Kaimal and Vijayabalan, 2015).

Other synthetic waste oils such as waste lubrication oil or tire pyrolysis oil, as well as bio-based municipal waste, olive mill, or kapok waste oils, have also been proposed for biofuel production (Yadav et al., 2015).

The aim of this section is to define the state of the art for bioethanol production worldwide based on the raw materials employed and the sustainability of the industry; thus description and challenges in processing, industrial methods, metabolic pathways, genetically engineering strains, etc., have been already extensively covered by different authors (Luque et al., 2011; Koutinas et al., 2014).

In 2014, two of the most important bioethanol companies worldwide, the Andalusian Abengoa BioEnergy (Spain) and the American POET-DSM, opened commercial-scale cellulosic bioethanol facilities for the first time in the United States (Association, 2015). Mendota Bioenergy LLC operates also in the Wissington sugar factory (California, US), using local sugar beet including lignocellulosic residues for the production of ethanol.

Research and development companies such as the Spanish Alkol Biotech, the Brazilian Granbio, or the American Arcadia Biosciences Inc. focus on the development of hybrid energy crops (eg, energy cane) with high yields, to use sugar beet for the production of cellulosic ethanol, as well as on nitrogen-use efficiency to reduce consumable inputs.

Sweet sorghum biorefineries are also a target for the industry with the implementation in 2013 of a 20 million gallons per year (MMGY) bioethanol facility in Florida, US, and the launch of “high-biomass-sorghum” strains from the biotech companies NexSteppe and Ceres, to be used in US, Brazil, and Europe (Platform, 2015).

The Chinese government is also aware of the sustainability problem related to Chinese biofuels, targeting the production of 300 million tons of cellulosic and nongrain-based ethanol by 2020. Even though, considering present challenges in raw material transport and slow progress on cellulose valorization processes, along with the impossibility of private companies to receive government incentives and subsidies, most experts assume the production of only 10 million tons by the date target (Ji, 2015).

The Thai Roong Ruang Group recently opened a second production line using cane bagasse in its already-existing molasses-based ethanol plant. The project for second-generation biofuel production is still in the experimental stage, producing only 10,000 L/day, due to high production costs compared to first-generation ethanol from cassava roots and molasses. Low petroleum prices will negatively affect the survival of this revolutionary project in Thailand (Prasertsri, 2015).

Moreover, lignocellulosic biomass is a source of energy beyond the bioethanol industry. For example, the wood-processing industry is an enormous market that generates approximately 131,388,000 m³ of wood residues every year. Koutinas et al. evaluated the potential of wood-based waste, along with pulp and paper mills residues and food industry waste streams, for the production of sustainable biofuels and chemicals (Koutinas et al., 2014). Valorization of lignocellulosic raw materials for the production of biofuels is described later in this book and can be consulted in chapter “Algae for biofuels: an emerging feedstock.”

As an example for the industrial application of waste valorization, the enterprise Enerkem opened in Canada a 5-million-liter-capacity demonstration bioethanol and biochemical plant in 2012 based on wood. The same company finished in 2015 the construction of a larger plant (30 million liter) in Edmonton, Alberta, for the production of lignocellulosic ethanol from municipal solid waste. Despite this, production is nowadays focused on methanol, carbon dioxide, and other chemicals that present higher revenues than ethanol due to their lower market prices (Dessureault, 2015). Future plants for the production of cellulosic ethanol from nonrecyclable wastes have also been announced in Quebec, while other Canadian cities will produced clean bio-based heat and power through gasification, pyrolytic bio-oil, etc.

Indian biofuel public and private industries also concentrate their efforts on the production of advanced biofuels from lignocellulosic materials from wood and forest waste as well as agricultural waste (eg, corn cob, bagasse, stalk, and forage crops). This industry still has some years ahead to demonstrate its capacity to get over the technological challenges presented and to prove large-scale economic viability. Sugar bagasse, an adaptable, carbon-neutral, abundant vegetable raw material in India, may be used for the production of heat and power; thus nowadays it is already used as a fuel in sugar mills, rice mills, textiles, etc. Its utilization would promote rural development with great social benefits. Power generation in India relies also on other biomass materials, such as bagasse, rice husk, straw, cotton stalk, coconut shells, soy husk, de-oiled cakes, coffee waste, jute wastes, peanut shells, or sawdust. The availability of biomass in India, from both agricultural and forestry-wasteland residues, is estimated at 915 million metric tons, which implies great valorization possibilities for a developing country (Aradhey, 2015).