**5. Potential of pyrolysis in biowaste conversion**

Pyrolysis is the decomposition of organic materials using heat in the absence of oxygen to produce solid, liquid, and gases [51, 52]. It is said to have lower operating temperatures and emissions of air pollutants compared to combustion and gasification [52]. Pyrolysis has emerged as technology for current and future conversion technology of biomass [53]. The type, quality, and quantity of product depend on how operating parameters are controlled [52]. Its ability to treat almost all types of organic and production of solid, liquid, and gas fuel has made it a prospective technology of biomass conversion. The innovations on reactors, catalysts, and upgrading technologies have increased the importance of pyrolysis. Its level of development has made it a prospective technology for biowastes especially lignocellulosic biowastes. Therefore, pyrolysis is the current and future appropriate technology in the conversion of biowastes in Africa.

### **5.1 Categories of pyrolysis**

Depending on the heating rate, operating temperature, and product distribution, pyrolysis can be classified as **Table 4**. It can be seen that slow pyrolysis targets char, fast and flash pyrolysis target targets bio oil while intermediate targets all. Intermediate pyrolysis has emerged as an alternative pyrolysis that is flexible, modular, economic, and ability to handle different biomass materials. It is appreciated to favor well the treatment of biowastes (**Table 4**).

The quality and distribution of products depends on a well control of parameters. Among important parameters as such as feed composition, temperature, heating rate, catalysts, feed residence time, vapor residence time, particle size, and moisture contents. The composition of feedstock should be made of organic components to lead to effective conversion. For example, cellulose and hemicelluloses produce high bio-oil while lignin can yield up to 40% of its weight as char [57]. The presence of non-organic materials reduces the amount of useful products.

Temperature also dictates the composition and yield of the products [58]. Generally, an increase in temperature increases the amount of bio-oil and gas. Higher pyrolysis temperature favors production of hydrogen, while low temperatures

*Biowastes as a Potential Energy Source in Africa DOI: http://dx.doi.org/10.5772/intechopen.99992*


**Table 4.**

*Classification of pyrolysis processes [54–56].*

produces more char [59]. The increase in temperature reduces the amount of char but increases its quality due to decrease of the volatile matter in char.

The residence time determines the quality of the products. For example, longer residence time at low temperature favors production of biochar and its quality increases due to favoring the development of micro-and macro-pores of bio-char [60, 61]. Reduced residence time reduces the re-polymerization thus reducing the amount of char [62]. Heating rate affects the quantity, quality, and composition of products. Rapid heating gives higher volatiles and more small reactive char than those produced by a slower heating process [52]. Slower heating rate increases the amount of char due to the secondary char produced from a reaction between the primary char and the volatiles [57].

Water content affects the quality and quantity of products. It promotes the reduction of species in bio-oil and improves the production of light aromatic [63]. Water can also catalyze char formation by acting as steam activator [64, 65]. Large amount of water reduces energy content in the feed; and hence, an optimal content is required. Particle size affects the quality and quantity of products by affecting the heat transfer. The decrease in size increases heating rate and easy escape of condensable products. In addition, small size favors liquid formation, hydrogen and carbon monoxide; while large size favors the formation of char and its quality due to secondary cracking [66–68].

Inert gas carrier (sweeping gas) controls the vapor residence time. Higher flow rates cause rapid removal of products that leads to minimization of secondary reactions such as char formation, thus increasing gas production [67, 69]. Poor properties of bio-oil such as high viscosity, non-volatility, high acidity, corrosiveness, instability upon storage, lower energy density, and immiscibility with fossil fuels caused by the presence of oxygen can be improved using catalysts [70]. The catalyst increases the quality of products through increased cracking, selectivity, and deoxygenation [71]. This increases the quality of bio-oil and biochar while decreasing their quantities [51, 72]. It has been reported that, the presence of catalyst *HZSM* −5 reduced the amount of oxygenated compounds and thus increased the quality of bio-oil [72, 73]. Studies have also found out that bio-char production increases with an increase of *NaOH*, *NaCl*, *CaO*, and *Na SiO* 2 3 [74, 75].

### **5.2 Pyrolysis in biowaste conversion**

Pyrolysis has emerged as the appropriate technology in biowaste conversion although its applicability in household in difficult [52]. It can convert almost all materials; small and large size, lignocellulosic and non-lignocellulosic, wet and dry, and variety of compositions. It has the ability to produce usable product with little or no upgrading. For example, intermediate and fast can produce bio oil which can be used with little or no upgrading. Hydrothermal pyrolysis can convert wet materials to produce usable solid and liquids. Its high reaction rate compared to anaerobic digestion leads to small reactor. Its main challenges to be used biowaste conversion are the investment and operation cost, sensitivity of the process, and difficulty in operation. For Africa, this becomes a promising technology but difficult conversion technology for small and household users.
