**3. MSW as a source of energy**

Solid waste is composed of a range of constituents, which are not all recyclable/ reusable [14]. Due to ever increasing quantities of solid waste generated throughout the world and the fact that approximately 70% of it is organic, it is regarded as a huge source of renewable energy [14, 15]. In addition, MSW is abundant, free and available everywhere, recovering energy from it has gained much attention, globally, with the developed countries carrying the process to the full capacity of the technology [16]. MSW presents various drawbacks during incineration. These include low energy content approximately 10–13 MMBTU/ton, an amount which is far less than sub-bituminous coal which has about 17–21 MMBTU/ton [17]. In addition, solid waste has a high moisture content approximately >50%. Recovering energy from waste simply means that huge supply of energy is required for igniting and drying the waste for combustion to take place. Its ultimate composition is extremely heterogeneous making combustion very difficult and unstable and different municipalities can use completely different waste pre-sorting approaches. Despite the heterogeneity nature of its ultimate composition, its elemental composition is also diverse. Levels of chlorine, sodium, potassium, sulfur, nitrogen, heavy metals (e.g., zinc, lead, cadmium and others) and ash content in waste are greater than those in lingo-cellulosic feedstock [4]. When combusted, these elements react with oxygen or react among themselves producing pollutants e.g., oxides, chlorides, dioxins and furans. Incineration plant can be affected by these pollutants, it need frequent clean-up [17]. Apart from that, MSW is occurs everywhere on land and collecting it to one point is a challenge and costly [14]. In general, densely populated areas produce substantial amounts of MSW and incineration plants can be installed in such areas. Locations which are less populated require new, smaller-scale technologies that need small amounts of waste as feedstock [14, 17].

### **3.1 Energy recovery from waste incineration**

MSW is a complex assortment of combustible materials such as food waste, paper, yard trimmings, plastic, as well as sludge from wastewater treatment and incombustible materials including metals, glass, rags, construction and demolition waste which are intermingled [16]. **Table 1** shows the ultimate analysis of the MSW fractions. The most preferred approach of recovering energy from MSW is incineration. If the amount of energy that can be produced from the recovery process is greater than R1 formula, then waste energy recovery is viable but if the amount of energy is less than R1 formula then waste should be disposed of in landfills [18]. Incineration technology has the advantages of killing pathogens as well as fast-volume and mass reduction of waste by approximately 90% and 70%, respectively, with the residual waste going to the landfill [19]. Because incineration plant is generally installed close to the source of waste, it also reduces the distance of hauling municipal wastes to the plant. Although some authors [16, 20] point that, these advantages are equipoised by emissions



**Table 1.** *Ultimate analysis of MSW fractions.* of oxides of sulfur and carbon, heavy metals, particulates, and dioxins where it is estimated that each ton of MSW incinerated produces approximately 15–40 kg of hazardous waste, others [1, 21] argue that recent incineration technologies are equipped with sophisticated air pollution cleaning system where pollutants emitted are within the standards [16]. Nevertheless, incineration technology is still claimed to produce dioxins and furans into the environment which are toxic but indeed there is still no report that is available of anyone who have been affected by dioxin from current incinerators.

The most used facilities for the incineration of waste are modular systems, mass burn as well as refuse derived fuel systems. In the mass burn technologies, waste is converted to energy through the mass combustion process. The waste is incinerated as received with or without prior sorting before entering the burning combustion chamber [22]. This technology burn waste in an incineration chamber supplied with surplus air which stimulates the complete mixing of waste with air and causes turbulence with the chamber to ensure that there is homogenous mixing for complete combustion. This homogenous mixing is vital owing to the heterogeneity of solid waste. Mass-burn technology burns waste on a sloping, moving grate that vibrates to shake the MSW mixing it with air [23]. In the modular Systems, waste is combusted without being processed. This technology is different from mass burning because they are smaller and movable from one place to another. In refuse derived fuel (RDF) technologies, MSW is shredded by the mechanical methods. This removes incombustible materials from the combustible fractions thus increasing the heating content of the waste which can be used as fuel in RDF furnace [22].

The number of waste to energy incinerators has amplified from an approximately 200 to over 800 plants worldwide now [24]. With a mean calorific value of 10 MJ/kg from the MSW, it is estimated that approximately 500 kWhe/ton waste of energy is produced from the combustion of waste. This amount of energy can be transformed to usable energy forms such as heat and electricity. The energy generated maybe increased by elevating the steam parameters i.e., steam temperature and steam pressure. However, this is hindered by (i) high-temperature corrosion of the boiler superheater tubes, (ii) fouling and ash deposition on tubes which then reduces the heat transfer to the steam from the flue gas, and (iii) fluctuation in steam temperature [24, 25]. Controlling these challenges results in the sustainable treatment of waste.
