**3.4 Pyrolysis**

**158** Pyrolysis is the thermochemical degradation of organic waste at high temperatures with no oxygen [24, 37, 38]. Also, the process is usually powered by the energy *Evaluating Waste-to-Energy Technologies as a Waste Management Solution for Uganda DOI: http://dx.doi.org/10.5772/intechopen.101904*

**Figure 8.** *A schematic illustration of a typical incineration plant [35].*

**Figure 9.** *The Högdalen CHP-plant in Stockholm, Sweden [36].*

produced during thermal degradation (endothermic process) [38, 39]. It is reported that the process requires consistent feedstock which limits its commercial-scale applicability from accepting MSW since MSW in its raw form is usually not suitable for pyrolysis and normally would need pre-treatment through mechanical preparation and separation to remove inert materials as well as glass, and metals [6]. However, the process is gaining more attention than incineration because of its ability to use a vast range of industrial and domestic waste and its ability to generate different products

[37]. The by-products are either gases (syngas), liquids (bio-oil), or solids (bio-char) and the process comprises a secondary chamber that where the gases or oils are burned to generate electricity or usable heat [24]. **Figure 10** presents a schematic of pyrolysis plant and the variation in yield depend on parameters such as heating rate, the pyrolytic temperature, and evacuation of the product from the reaction zone [37, 40, 41].

The biochar comprises non-combustible components plus carbon while syngas comprises combustible matter that is CO, H, H4, and other volatile organic compounds. The bio-oil has high heat value and is used as industrial fuel oil [6, 19]. Also, the products can be a fuel to generate power using gas engines and gas turbines [24]. The CVs for syngas, bio-oil, and biochar range from 10–15 MJ/Nm3 , 15–20 MJ/Nm3 , and 34 MJ/kg, respectively. Even though char's CV is comparable to coal, it is limited by the complex nature of waste which might comprise hazardous elements that pose risk to humans and the environment and care must be taken [37]. Nonetheless, the products are ready to use and specifically, the waste polymers generate the best oil product.

The pyrolysis technology is expensive compared to commercial ways of treatment [24] and the need to pre-treat waste. The pre-treatment devices are expensive and complex [37]. Also, the syngas causes tarring which can easily lead to blockages and operational challenges. Because of this, pyrolysis facilities have been associated with failures and inefficiencies [6]. Failure to sort waste before the process could lead to the production of dangerous nitrogen compounds in the products hence the need gas cleaning devices [37].

**Figure 11** shows a pyrolysis plant located in Bulgaria that converts plastic waste into diesel oil [42] and despite the various pilot plants and industrial-scale developments, it is reported [37] the process is still not economically viable.
