**4. Problems faced during incineration**

Incineration is the most preferred method for the treatment of waste and it releases energy which can be utilized for heat and electricity generation. Due to the high chloride, alkali and sulphate content in the MSW, the technology suffers grave high temperature corrosion, ash deposition, and fluctuation in steam temperature thereby limiting efficient utilization or treatment of waste [24].

#### **4.1 High temperature corrosion**

Corrosion that occur at elevated temperatures in waste to energy technologies has been extensively investigated and the attack is caused by large amounts of chlorides and sulphates that appear in the flue gas of the combustion gas. Chloride are more dominant in inducing the corrosion attack than sulphate in waste to energy plants due to the elevated chlorine concentrations in MSW. HCl and Cl2 generated are emitted

#### *Efficient Treatment of Municipal Solid Waste in Incinerators for Energy Production DOI: http://dx.doi.org/10.5772/intechopen.108449*

into the flue gas as the combustion proceeds, where HCl occurs in bulk gas containing moisture, while Cl2 occur in a dry environment, and may also result from the decomposition of HCl [26]. The chlorine-induced corrosion mechanism is generally understood as the active oxidation process. The main corrosion reactions taking place in the active oxidation mechanism, are illustrated in reactions (1)–(5) and in our review paper [1]:

$$2\text{HCl}(\text{g}) + \text{z}/2\text{O}\_{\text{z}}\text{(g)} = \text{Cl}\_{\text{z}}\text{(g)} + \text{H}\_{\text{z}}\text{O}(\text{g})\tag{1}$$

$$\text{M(s)} + \text{Cl}\_{\text{z}}\left(\text{g}\right) = \text{MCl}\_{\text{z}}\left(\text{s}\right) \tag{2}$$

$$\text{MCl}\_{\text{z}}(\text{s}) = \text{MCl}\_{\text{z}}(\text{g}) \tag{3}$$

$$2\text{MCl}\_{\text{z}}\left(\text{g}\right) + \text{g}/2\text{O}\_{\text{z}}\left(\text{g}\right) = \text{M}\_{\text{z}}\text{O}\_{\text{y}}\left(\text{s}\right) + 2\text{Cl}\_{\text{z}}\left(\text{g}\right) \tag{4}$$

$$\text{\textbullet{\text{FeCl}\_{z}(g)} + 2\text{O}\_{z}(g) = \text{Fe}\_{\text{\textbullet}}\text{O}\_{4}(s) + \text{\textbulletCl}\_{z}(g) \tag{5}$$

Where M = Fe, Cr or Ni.

When the conditions inside the boiler have plenty supply of oxygen and temperatures is less than 600°C, HCl(g) become oxidized at the deposit/gas interface, giving Cl2 gas. However, when the temperature is greater than 600°C while moisture is present, HCl generation is promoted [27]. Chlorine that is produced volatises and evaporate through the pores and cracks to the scale/metal interface, where oxygen partial pressure is low, and reacts with the metal elements producing solid metal chlorides. Due to high volatility, metal chlorides evaporate continuously and diffuse to the gas-oxide boundary and react with oxygen thereby converted to oxides forming a porous oxide layer that cannot prevent further inward diffusion of chlorine gas to the metal substrate for corrosion attack. The chlorine released diffuses back to the metal surface; therefore, a cycle that, with little or no net depletion of chlorine, provides a continuous removal of metals away from the metal surface, toward regions with higher oxygen partial pressure, where oxides are formed [28].

While the active oxidation process has credibly received experimental support [28–32], some problems with the corrosion mechanism appear to have been ignored which led to the difficulties in actually finding the solution to the corrosion problem. The problems are that why the metal/scale interface has low oxygen partial pressure, but high for chlorine? Does this imply that oxygen is prevented to diffuse across the scale, but chlorine, which has a large molecular size, is allowed? Li et al. [33], suggested that the reaction of oxygen in the oxidation of alloy elements, particularly Cr, at the metal/scale interface, decrease the partial pressure of oxygen, while Cl2 continue diffusing into the interface leading to increased Cl2 partial pressure, at the metal/scale interface which in turn react with tube metals.

#### **4.2 Kinetic modeling of corrosion**

High temperature corrosion can be studied either by experiments (lab-scale, pilot-scale, and commercial-scale studies) or by theoretical studies which include kinetic modeling and also thermodynamic equilibrium as well as CFD modeling. The chemical reactions that occur in the gas phase between corrosive gases such as chorine,


#### **Table 2.**

*Composition of MSW.*

potassium and sulfur during combustion of MSW determine the species that reaches and reacts with the boiler tube surfaces [34]. **Table 2** shows the concentrations of elements that are found in MSW and that participate in corrosion process for both the lowest and the worst case scenarios. These gas phase and surface reactions are complicated and cannot be identified by experiments due to the inability of online measurements. Thermodynamic equilibrium calculations are normally used to predict the reaction products. Nevertheless, such predictions give products under conditions that are thermodynamically stable whereas in actual systems the local kinetics controls the corrosion process [38]. The main steps in corrosion include the diffusion of gaseous species from the combustion environment to the metal 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, and pressure gradients lead to the deviations of this flow [39]. The surface reactions involve the formation of intermediates products which further reacts with either gaseous species from the flue gas (Eley–Rideal mechanism) or with other adsorbed intermediates (Langmuir–Hinshelwood mechanism) [38]. Since experiments have the inability of online detection, theoretical calculations provide new and useful information regarding the corrosion phenomenon and optimisation of the combustion environment for reduced corrosion rates [39].

Ma et al. [40] developed a kinetic model for the surface reactions between gaseous K, Cl, and S species with pure Fe metal during corrosion. The model was employed to explore the effect of KCl on the corrosion of pure iron metal. The amounts of KCl used resulted in different K/S ratios. The authors observed that increasing KCl amounts, which increases K/S ratio, accelerated the corrosion of Fe metal as illustrated in **Figure 2**.

Chen et al. [41] further employed kinetic modeling to investigate the operating conditions that reduces the corrosion rate on the boiler tube metals. They investigated concentration of sulfur dioxide, moisture content, hydrochloric acid concentration and influence of steam pressure and temperature. The authors found that the concentration of sulfur plays a major role in reducing the corrosion effect. The concentration of SO2 between 0 and 270 ppm resulted in less effect on corrosion impacts. This was ascribed to small amounts of sulfur in the combustion system which could not induce any effect. However, the concentration between 270 and 500 ppm resulted in a reduced corrosion rate of the boiler tube metal. The author put forward that the concentration of sulfur was sufficient to transform the corrosive potassium chloride to less corrosive potassium sulphates. The sulfur contents increased the reaction

*Efficient Treatment of Municipal Solid Waste in Incinerators for Energy Production DOI: http://dx.doi.org/10.5772/intechopen.108449*

**Figure 2.** *Influence of KCl on corrosion of iron metal [40].*

activity for KOH + SO3(+M) = KHSO4(+M), which also has a reverse reaction. This on-set a reaction loop where potassium is sulfated to KHSO4, decompose back to KOH and sulfated again. The continuation of the cycle eventually leads to sulphation of all the available K due to K/H exchange between KOH and KHSO4 to K2SO4. Sulfate formed prevents reactions between Cl and Fe, thus corrosion decreases. A concentration above 500 ppm was observed exacerbates the corrosion impact. Chen et al. [41] explained the mechanism by saying that when FeS is formed, and because there are cation vacancies in FeS structure, there is high diffusion rate of Fe ions through FeS facilitated by iron concentration gradient established by more stable iron oxide formed at the exterior surfaces leading to increased mass loss [42]. The influence of moisture was investigated and it was observed that moisture content ~10vol. % resulted in a reduced the corrosion rate. The inhibitory effect of H2O vapor on corrosion rate was due to the facilitation of the formation of K2SO4 [41]. The corrosion mechanism of superheater made up of pure iron is shown in **Figure 3**.

#### **4.3 Ash deposition**

During combustion of waste, ash particles of various size migrate from the combustion bed to the superheater surfaces where they form ash deposits. Ash build-up during is divided into two mechanisms which are influenced by the flue gas temperature in the boiler. The first one is via solidified slag formation and the second one is the powdered ash depositions. The solidified slag deposition is formed between 1070 and 1320K and typically has high contents of Fe2O3 and sulfates and low contents of SiO2 and Al2O3. The powdered ash deposition occurs below 1070K and contains more than 50% SiO2 and over 20% Al2O3 [43].

After reaching the boiler tube surfaces, the incident gases heterogeneously react with the tube surface, resulting in the collection of mass of the ash deposit. Chemical reaction cause the vapor pressure of a species to be zero on the deposit surface, hence keeping a concentration gradient with the bulk gas which continuously allows the diffusion processes and reactions to occur [44]. The most key reactions that facilitates

**Figure 3.** *Corrosion mechanism of pure iron metal.*

the ash deposition and growth are alkali absorption, sulphation, oxidation or reduction, carbonation and creation of eutectics due to reaction of K, Na, Fe, Ca, Si, and Al [1]. The main species causing sulphation are compounds of alkali metals, such as potassium and sodium hydroxides and chlorides. This was evidenced by Hansen et al. [45], who observed a potassium sulphate layer in the deposits, which were exposed to the waste combustion conditions for over a period of 1 year. On the contrary, the probes which were exposed for a short period of times contained only KCl without K2SO4. Therefore, it is concluded that KCl is the species that initiate the ash deposition by forming eutectic mixtures with FeCl2 at 355°C, with K2SO4 at 690°C, and also with FexOy. The eutectic mixture then sticks on the surfaces of the boiler tube then followed by reaction with sulfur-containing species, such as gaseous SO2 or SO3 forming potassium sulphate. Deposition forms when ash accumulates on the heating surface at temperatures lower than the ash fusion point. Detailed ash deposition mechanism can be found in the [46]. Once the ash deposits are formed, they induce solid state high temperature corrosion of heat exchange surfaces, reduce heat exchange from the flue gas to the heated medium and, hence, inhibit heat transfer in the boiler and reduce the boiler efficiency. Once the boiler efficiency is reduced, waste treatment rates becomes compromised.
