**2.4 Pyrolysis**

Pyrolysis is a thermochemical decomposition process that occurs in the total absence of oxygen and at relatively low temperatures (500–800°C) when compared to gasification (800–1000°C). There are different types of pyrolysis, each favoring the production of three different products: pyrogas, pyrolysis oil, and char (**Figure 4**). The relative proportions of each product depend on the applied pyrolysis method, the type of feedstock, and temperature. Archeological evidence suggests that during the Middle Paleolithic, Neanderthals resorted to pyrolysis to produce a kind of tar which they would use as glue. The use of this process in the production of all types of products was widespread throughout the world until the beginning of the twentieth century. Nowadays, pyrolysis is once again being viewed as one interesting solution to produce energy, fuels, and chemicals using local wastes.

The major advantage of pyrolysis in waste recovery may be in being able to convert low-energy-density materials into high-energy-density products. As an example, pyrolysis has been adopted as an alternative to the treatment of plastic wastes to produce plastic-derived oil (PDO) [31] and pyrogas [32]. PDO has been reported to be similar to diesel (C13–C20) [33]; however, additional processing is needed to deal with aromatic compounds. The use of calcium carbonate (CaCO3) in the pyrolysis of horse manure allows for lower temperatures due to the catalytic effects of CaCO3 as a possible source of CO2 [34]. Co-pyrolysis of different plastic mixtures [35], as well as the use of catalysts [36–39], has also yield interesting results concerning the productivity and quality of the PDO components.

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mentation, and its evolution.

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

materials for further chemical processing.

desulphurization of the bio-oil [48].

**2.6 Torrefaction**

**2.5 Hydrothermal liquefaction (thermal depolymerization)**

Hydrothermal liquefaction or thermal depolymerization is the thermochemical conversion of solid waste into a liquid using moderate temperatures (250–375°C) and high pressures (4–22 MPa). Similar to pyrolysis but occurring with the waste immersed in water at high pressures and temperatures, the process leads to the break of long carbon chains, resulting in a bio-oil with a good calorific value. As a technological option, the process does not need catalysts, but research has indicated that the use of alkaline catalysts allows the formation of high-value chemicals. Hydrothermal liquefaction is attractive because efficiencies greater than 80% are common when converting biomass into fuels and other high-value chemicals [40]. This technology has enormous potential, particularly to produce biofuels and raw

The concept of hydrothermal liquefaction was first explored in the 1920s and was further developed in the 1950s by H. Heinemann. However, only after the oil crisis in the 1970s did the first efforts to exploit this technology finally emerged, being the concept finally proved at pilot scale with the construction of Biomass Liquefaction Experimental Facility in Oregon, USA [41]. Recently, research regarding this technology has focused on finding new catalysts and developing novel ways of converting the produced bio-oils into high-value products. In practice, hydrothermal liquefaction is valued because it provides rapid conversion of waste biomass into bio-oil, avoiding the high energy cost of drying [42]. Most studies have shown that temperatures between 250 and 370°C are optimal for the production of bio-oil, with no general conclusion given about the effects of reaction time and moisture content [43]. Hydrothermal co-liquefaction is an interesting pathway and should be explored in future studies [44, 45]. Both the addition of potassium carbonate (K2CO3) [46] and the reuse of the liquid were reported to increase calorific value and productivity. The addition of solvents was also observed to enhance the process [47], while the addition of metallic catalysts led to deoxygenation and

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 imple-

**Figure 4.** *Example scheme of pyrolysis technology.*

*Elements of Bioeconomy*

**2.4 Pyrolysis**

local wastes.

components.

integration in second-generation biorefineries.

biopolymers accessible for further treatment via other processes such as fermentation, hydrolysis, anaerobic digestion, and densification. The production of biogas by anaerobic digestion using lignocellulosic wastes, for example, is considered a huge challenge due to its recalcitrant nature (non-biodegradability) [27]. In this regard, the use of explosive steam decompression as a form of pretreatment has been proven to enhance the production of biogas. Moreover, ethanol production and syngas production using lignocellulosic feedstocks have also been reported to proceed with higher calorific value and lower temperatures, respectively, when precluded with steam explosion [28, 29]. A promising solution for continuous steam explosion has been presented by a research team from South China University of Technology [30] allowing for process scale-up and its potential

Pyrolysis is a thermochemical decomposition process that occurs in the total absence of oxygen and at relatively low temperatures (500–800°C) when compared to gasification (800–1000°C). There are different types of pyrolysis, each favoring the production of three different products: pyrogas, pyrolysis oil, and char (**Figure 4**). The relative proportions of each product depend on the applied pyrolysis method, the type of feedstock, and temperature. Archeological evidence suggests that during the Middle Paleolithic, Neanderthals resorted to pyrolysis to produce a kind of tar which they would use as glue. The use of this process in the production of all types of products was widespread throughout the world until the beginning of the twentieth century. Nowadays, pyrolysis is once again being viewed as one interesting solution to produce energy, fuels, and chemicals using

The major advantage of pyrolysis in waste recovery may be in being able to convert low-energy-density materials into high-energy-density products. As an example, pyrolysis has been adopted as an alternative to the treatment of plastic wastes to produce plastic-derived oil (PDO) [31] and pyrogas [32]. PDO has been reported to be similar to diesel (C13–C20) [33]; however, additional processing is needed to deal with aromatic compounds. The use of calcium carbonate (CaCO3) in the pyrolysis of horse manure allows for lower temperatures due to the catalytic effects of CaCO3 as a possible source of CO2 [34]. Co-pyrolysis of different plastic mixtures [35], as well as the use of catalysts [36–39], has also yield interesting results concerning the productivity and quality of the PDO

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**Figure 4.**

*Example scheme of pyrolysis technology.*
