**2.6 Torrefaction**

Torrefaction is a form of thermal treatment which takes place between 200 and 500°C in the absence of oxygen. As temperature rises, moisture and superfluous volatiles are gradually released, and biopolymers such as hemicellulose, cellulose, and lignite are partially decomposed, depending on process conditions [49]. At mild temperatures (235–275°C), for example, the degradation of hemicellulose is accelerated, and the release of the volatiles is intensified, while cellulose is only consumed to some degree. On average, the process results in mass losses and decreases in calorific value (20% and 10%, respectively) but yields a more homogeneous waste composition and leads to higher energy densities. Some biomasses have characteristics that hinder their utilization as energy feedstocks; using this process as pretreatment allows the use of a broad spectrum of wastes in other WtE technologies. The main product of torrefaction is, therefore, a waste with improved characteristics regarding its energy use. More than 150 torrefaction installations worldwide with powers from 50 to 700 MWe have successfully tested the co-combustion of torrefied biomass, reducing greenhouse gas emissions and dependence on fossil fuels. It is expected that torrefied biomass could represent 5–10% of industrial applications in Europe [49]. However, the market for torrefied waste products is still very recent, and there is not enough data available about the real use of technology, its implementation, and its evolution.

Among researchers, torrefaction has been viewed as an excellent pretreatment for improving the energy recovery features of several wastes creating products with low oxygen to carbon ratios and high calorific values for co-gasification and cocombustion applications [50]. As an example, the torrefaction of several pomaces [51] and prunings [52] led to very promising results with calorific values increased to near lignite levels. Other interesting results have been reported by researchers [50] dealing with the very heterogeneous nature of MSW which along with high moisture contents make them challenging for application in WTE and WTL processes. Most studies reported a positive correlation between the calorific value and torrefaction temperature.

### **2.7 Anaerobic digestion**

Anaerobic digestion (AD) consists in the conversion of biodegradable organic matter in the absence of oxygen in which a biogas rich in methane is produced [53]. Typically, the resulting biogas is composed of 50–75% CH4, 25–50% CO2, and 1–15% of other gases such as H2O, NH3, and H2S. Another by-product of anaerobic digestion is the digestate, an excellent organic biofertilizer. Virtually all types of organic matter have the potential to be digested anaerobically to produce biogas. The most common organic wastes used in AD are agricultural, livestock industry, agroindustry, and municipal solid wastes and wastewater. Woody materials are less suitable because they contain a high proportion of lignite, making it very difficult to decompose biologically.

As a technology, AD is already mature and well developed. Since 2009, the number of biogas plants has greatly increased in Europe with biomethane production growing in line with sector development. In 2016 alone, energy production derived from biomethane increased by 4971 GWh (+40%) within the European countries reviewed [54]. The key to future research is thus the optimization of process parameters that affect efficiency. Temperature change, for example, is known to affect microbial activity and growth rates. Higher digestion temperatures, for instance in the thermophilic range, have been demonstrated to lead to higher biogas productivities, but thermophilic digestion represents a higher investment due to energy costs. On the other hand, digestion of simple substrates often results in a nutrient imbalance that affects the stability of the process. Thus, C/N ratio optimization by co-digestion has been widely tested with good results taking advantage of the synergies between different substrates. This strategy represents the most economical way to improve process productivity nowadays. The use of multiple steps in AD has also been observed to be an interesting solution for achieving the best use of different substrates [55–58].

The integration of anaerobic digestion with microalgae cultivation presents potential benefits [59, 60]. From an economic point of view, costs can be substantially reduced by using the digestate from AD as a source of nutrients for algae growth. However, several barriers will have to be overcome before the scale-up of the process. The main obstacle identified in the reviewed research was the need to find a robust microalga strain capable of binding with organic and inorganic carbon and tolerate extremes of pH.

#### **2.8 Fermentation**

Fermentation is an anaerobic metabolic process, in which microorganisms (bacteria, yeast) turn carbohydrates into fatty acids, alcohols, and gaseous products such as H2 and CO2 (**Figure 5**). The most common industrial products resulting from fermentation are ethanol, acetic acid, and citric acid (2-hydroxypropane-1,2,3-tricarboxylic

**41**

(13.2 g L<sup>−</sup><sup>1</sup>

**Figure 5.**

rate of 0.012 h<sup>−</sup><sup>1</sup>

**2.9 Enzyme treatment**

*Review of Biofuel Technologies in WtL and WtE DOI: http://dx.doi.org/10.5772/intechopen.84833*

*Example scheme of fermentation technology.*

acid). The conversion of sugars into ethanol is the most well-known form of fermentation, producing alcoholic beverages such as wine, beer, and cider. Interestingly, the same fermentation occurs in the production of bread, yogurt, and other foods fermented by the formation of lactic acid (2-hydroxypropanoic acid). In addition, there have been significant advances in the production of bioethanol, biobutanol (butan-1-ol), and bio-hydrogen (molecular hydrogen), among other high-valued chemicals. Continuous fermentation of syngas using fixed-bed drip reactors for ethanol production has been proven as a valid concept with the highest ethanol concentration

has encountered some difficulties in its development on an industrial scale. Besides fixed-bed bioreactors, other efforts related with reactor design have been focused on membranes combined with the formation of biofilms due to enhances in mass transfer. Studies on the production of bio-hydrogen have been focused on bio-photolysis of water using algae and cyanobacteria, photodecomposition of organic compounds by photosynthetic bacteria [62], and dark fermentation of organic compounds with anaerobes [63]. For dark fermentation, special attention has to be given to inhibitors such as the excess of substrate, micronutrients, macronutrients and metal ions, high temperatures, acidic pH levels, and competition from other microorganisms [63].

Enzymes are macromolecular biological catalysts which accelerate chemical reactions. In 1897, Eduard Buchner resorted to enzymes extracted from yeasts grown in his lab to ferment ethanol, a seminal work for which he received the Nobel Prize for Chemistry in 1907. Industrially, their application lies either in converting substrates into greater value products or as pretreatment for energy recovery and biofuel production. Nowadays, nearly all types of commercially available enzymes are produced by fermentation, being part in virtually every aspect of our lives, from the pharmaceutical industry to laundry detergents. In 2016, an industrial unit including an enzymatic pretreatment started to operate within a perspective of energy recovery from MSW. Specifically, enzymes degrade a fraction of the organics present in MSW so that they can be easily digested anaerobically. The facility is located in Northwich, England, and produces 5 MWe consuming 15 Mg h<sup>−</sup><sup>1</sup>

[64]. Another commercial application with good future perspective is enzymatic saccharification which can be used to produce bioethanol at a low cost. Some studies on bioethanol production from bamboo, for example, indicate that increasing the amount of the enzyme yields little improvement in the process highlighting

) obtained during co-current continuous syngas fermentation at a dilution

[61]. However, despite being a promising technology, the process

of MSW

*Review of Biofuel Technologies in WtL and WtE DOI: http://dx.doi.org/10.5772/intechopen.84833*

*Elements of Bioeconomy*

and torrefaction temperature.

**2.7 Anaerobic digestion**

decompose biologically.

different substrates [55–58].

and tolerate extremes of pH.

**2.8 Fermentation**

Among researchers, torrefaction has been viewed as an excellent pretreatment for improving the energy recovery features of several wastes creating products with low oxygen to carbon ratios and high calorific values for co-gasification and cocombustion applications [50]. As an example, the torrefaction of several pomaces [51] and prunings [52] led to very promising results with calorific values increased to near lignite levels. Other interesting results have been reported by researchers [50] dealing with the very heterogeneous nature of MSW which along with high moisture contents make them challenging for application in WTE and WTL processes. Most studies reported a positive correlation between the calorific value

Anaerobic digestion (AD) consists in the conversion of biodegradable organic matter in the absence of oxygen in which a biogas rich in methane is produced [53]. Typically, the resulting biogas is composed of 50–75% CH4, 25–50% CO2, and 1–15% of other gases such as H2O, NH3, and H2S. Another by-product of anaerobic digestion is the digestate, an excellent organic biofertilizer. Virtually all types of organic matter have the potential to be digested anaerobically to produce biogas. The most common organic wastes used in AD are agricultural, livestock industry, agroindustry, and municipal solid wastes and wastewater. Woody materials are less suitable because they contain a high proportion of lignite, making it very difficult to

As a technology, AD is already mature and well developed. Since 2009, the number of biogas plants has greatly increased in Europe with biomethane production growing in line with sector development. In 2016 alone, energy production derived from biomethane increased by 4971 GWh (+40%) within the European countries reviewed [54]. The key to future research is thus the optimization of process parameters that affect efficiency. Temperature change, for example, is known to affect microbial activity and growth rates. Higher digestion temperatures, for instance in the thermophilic range, have been demonstrated to lead to higher biogas productivities, but thermophilic digestion represents a higher investment due to energy costs. On the other hand, digestion of simple substrates often results in a nutrient imbalance that affects the stability of the process. Thus, C/N ratio optimization by co-digestion has been widely tested with good results taking advantage of the synergies between different substrates. This strategy represents the most economical way to improve process productivity nowadays. The use of multiple steps in AD has also been observed to be an interesting solution for achieving the best use of

The integration of anaerobic digestion with microalgae cultivation presents potential benefits [59, 60]. From an economic point of view, costs can be substantially reduced by using the digestate from AD as a source of nutrients for algae growth. However, several barriers will have to be overcome before the scale-up of the process. The main obstacle identified in the reviewed research was the need to find a robust microalga strain capable of binding with organic and inorganic carbon

Fermentation is an anaerobic metabolic process, in which microorganisms (bacteria, yeast) turn carbohydrates into fatty acids, alcohols, and gaseous products such as H2 and CO2 (**Figure 5**). The most common industrial products resulting from fermentation are ethanol, acetic acid, and citric acid (2-hydroxypropane-1,2,3-tricarboxylic

**40**

**Figure 5.** *Example scheme of fermentation technology.*

acid). The conversion of sugars into ethanol is the most well-known form of fermentation, producing alcoholic beverages such as wine, beer, and cider. Interestingly, the same fermentation occurs in the production of bread, yogurt, and other foods fermented by the formation of lactic acid (2-hydroxypropanoic acid). In addition, there have been significant advances in the production of bioethanol, biobutanol (butan-1-ol), and bio-hydrogen (molecular hydrogen), among other high-valued chemicals.

Continuous fermentation of syngas using fixed-bed drip reactors for ethanol production has been proven as a valid concept with the highest ethanol concentration (13.2 g L<sup>−</sup><sup>1</sup> ) obtained during co-current continuous syngas fermentation at a dilution rate of 0.012 h<sup>−</sup><sup>1</sup> [61]. However, despite being a promising technology, the process has encountered some difficulties in its development on an industrial scale. Besides fixed-bed bioreactors, other efforts related with reactor design have been focused on membranes combined with the formation of biofilms due to enhances in mass transfer. Studies on the production of bio-hydrogen have been focused on bio-photolysis of water using algae and cyanobacteria, photodecomposition of organic compounds by photosynthetic bacteria [62], and dark fermentation of organic compounds with anaerobes [63]. For dark fermentation, special attention has to be given to inhibitors such as the excess of substrate, micronutrients, macronutrients and metal ions, high temperatures, acidic pH levels, and competition from other microorganisms [63].

### **2.9 Enzyme treatment**

Enzymes are macromolecular biological catalysts which accelerate chemical reactions. In 1897, Eduard Buchner resorted to enzymes extracted from yeasts grown in his lab to ferment ethanol, a seminal work for which he received the Nobel Prize for Chemistry in 1907. Industrially, their application lies either in converting substrates into greater value products or as pretreatment for energy recovery and biofuel production. Nowadays, nearly all types of commercially available enzymes are produced by fermentation, being part in virtually every aspect of our lives, from the pharmaceutical industry to laundry detergents. In 2016, an industrial unit including an enzymatic pretreatment started to operate within a perspective of energy recovery from MSW. Specifically, enzymes degrade a fraction of the organics present in MSW so that they can be easily digested anaerobically. The facility is located in Northwich, England, and produces 5 MWe consuming 15 Mg h<sup>−</sup><sup>1</sup> of MSW [64]. Another commercial application with good future perspective is enzymatic saccharification which can be used to produce bioethanol at a low cost. Some studies on bioethanol production from bamboo, for example, indicate that increasing the amount of the enzyme yields little improvement in the process highlighting

the need for optimization depending on the waste to be transformed [65]. Other experiments have focused on process enhancement via salt pretreatment. Addition of inorganic salts, for instance, has been reported to improve reducing sugar yields of sugarcane leaf wastes and mustard stalk and straw [66, 67].
