**1. Introduction**

The growing population and economic development have resulted in increased municipal solid waste (MSW) generation which surpasses the current ability of the municipal authorities to handle it [1, 2]. Landfilling is the utmost preferred method [3] but the least considered method for waste disposal and management owing to the release of methane gas and discharge of the leachate into the ground water, thereby polluting it and rendering it unsafe for domestic purposes [4]. The emissions of methane and other greenhouse gases (GHGs) have induced and inflicted global warming which in turn affected the sustainable development of countries, particularly in the developing world. As a result, incineration of MSW in incinerators for energy

recovery has been considered as the most preferred method for sustainable waste management, and safe disposal of waste, because of the plusses of quick in mass and volume reduction by ~70% and ~ 90%, respectively, electricity and heat energy recovery, as well as complete disinfection [1, 5].

Currently, around 220 million tons of MSW are treated globally, in over 800 waste-to-energy (WtE) incineration plants [6]. Data showed that the total energy produced globally from municipal wastes in 2010 was 41.743 GWh, while in 2015 it had increased to 62.507 GWh [7]. This increase in energy generation indicates that combustion of MSW in WtE incinerators has drawn increasing attention. Aside from generating energy, WtE incineration plants have the benefit of lowering GHG CO2 emissions per unit of coal substituted, since waste fractions are largely biogenic approximately 70% are combustible organics materials [8–10]. In 2007, the worldwide impact of waste combustion in WtE incineration facilities on climate change was estimated at 40 million tons CO2-eq, compared to 700 million tons CO2-eq obtained from landfilling of waste, and in 2015 at 60 million tons CO2-eq compared to 800 million tons CO2 for landfilling [9]. With the policy of diverting waste from landfills toward incineration, it is expected to reduce approximately 92 million tons CO2 equivalence per year by 2030 which is approximately 8% of 1137 million tons CO2-eq, that is predicted to be reduced by 2030 [1, 11].

Nevertheless, wastes contains elevated chlorine concentration (0.45–1.00 wt. %) [10, 11] and salts of alkali metals especially Na and K, its incineration leads to grave fouling, ash deposition and high temperature corrosion of the boiler tube metals [11–16]. It has been estimated that chlorine-induced corrosion of the boiler tube metals reduces the electricity generation efficiency of WtE incineration plants by approximately 0.5–1.5% [17]. This occurrence forces the incineration plants to undergo an unplanned shut down for maintenance, causing loss in the treatment of wastes and reduced energy and electricity production. Generally, about 75% of the budget for the planned shutdown are consumed in the maintenance of the degraded boiler tubes in this unplanned downtime, resulting in economic loss [1].

The chemical reactions that occur in the gas phase between chlorine, potassium, and sulfur determine the species that reach and react with the boiler tube surfaces [18]. These gas phase and surface reactions are complicated and cannot be determined by experiments due to the inability of online measurements. Thermodynamic equilibrium calculations are normally used to predict the reaction products. Nevertheless, such predictions give only conditions that are thermodynamically stable whereas in actual systems the local kinetics controls the corrosion process [19]. The main steps in corrosion include the diffusion of gaseous species from the combustion environment to the surface, adsorption of the reactants onto the metal surface, reaction with the surface, desorption of the volatile products from the surface and diffusion of the products back to the combustion environment. The diffusion of gases is driven by the concentration gradient. However, convection, temperature gradients, and pressure gradients lead to the deviations of this flow [20]. The surface reactions involve the formation of intermediates which then reacts with further gaseous species from the flue gas [19]. The intermediates formed in both gas and surface reactions are difficulty to detect experimentally [21–23]. Therefore, it is necessary to study corrosion by combining theoretical calculations and experiments. Kinetic modeling reveals the fundamental nature of the reaction mechanisms [21]. The corrosion problem and other fireside challenges leads low MSW treatment efficiency.
