Efficient Treatment of Municipal Solid Waste in Incinerators for Energy Production

Abstract

MSW generation has increased drastically throughout the world surpassing the ability of municipalities to handle it. Treating waste in incinerators with energy recovery have been opted as an environmentally preferred method of waste management. However, waste incineration result in inefficient energy generation. The objective of this chapter is to provide a summary of issues leading to inefficient treatment of MSW and the potentials for improving it. High-temperature corrosion and ash-deposition on heat exchange surfaces are the major causes of inefficiency during waste incineration. Optimizing the operating conditions during incineration reduces the deterrent corrosion and ash deposition problems. The operating conditions can be optimized by conducting a kinetic modelling which identifies the conditions that reduces corrosion rate. These conditions are moisture content ~10 vol.% and SO2 ~250 ppm. Also, use of ecotubes and sergher-boiler prisms ensures high turbulence and mixing within the boiler which reduces the ash problems, thereby improving the incineration efficiency. Sorting of MSW using max Al robotic sorter and removal of alkali chlorides in waste through the use of sink-float process, centrifuge and hydrocyclone separation technologies lowers chlorine load hence lowering the severe ash problems and proves to be beneficial in improving the efficiency of treating MSW in incinerators.

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, 9, 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, 12, 13, 14, 15, 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, 22, 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.

Literature on hindrances and options to improve MSW incineration efficiency are scarce. Ma et al. [1] described the challenges occurred during incineration, however, that paper is not specifically on the options to improve waste incineration efficiency. Kumar and Samadder [12] reviewed technological options for effective energy recovery and the challenges faced in both developing and developed countries. The paper did not focus on methods to improve treatment efficiency. Prajapati et al. [13] summarized the recent technological advancements focusing on minimization of waste content and electricity generation focusing on biogas production. Varjani et al. [2] gave an overview of the existing sustainable MSW management technologies together with their limitations. While some attempts have conducted on the sustainable treatment of MSW, no study has elaborated on the option to improve the treatment efficiency. Understanding the details will assist in the design of the incinerator with less fireside challenges during combustion of MSW hence the treatment of MSW becomes sustainable. The purposes of the chapter was to shed light on the challenges that causes inefficient treatment of waste in WtE incinerators as well as the potentials for improving the efficient treatment of waste in incinerators. The results from this research will help incineration plant operators to implement and improve the efficiency of treating MSW, thereby becoming economically viable and competitive.

2. Methodology

The approach employed for retrieving the literature used in this chapter is summarized in Figure 1. Briefly, the literature was obtained from published articles found on academic databases which are Clarivate’s Web of Science®, ScienceDirect®, Researchgate®, Google Scholar®, and Scopus®. A Boolean search method employing the ‘AND’/‘OR’ were utilized in searching for literature upon entering the keywords. For example, the keywords used are MSW, incineration, high-temperature corrosion, ash deposition, combustion efficiency and kinetic modeling. A thorough cross search from the reference listed in the relevant articles were conducted for their significance to the current chapter.

Figure 1.

Flow diagram for content retrieval.

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 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.

Component	C	H	O	N	Cl	S	K	Na	Zn	Pb	Ash	Moisture	Ref
	wt.%					mg/kg					wt.%
Food items
(Mixed) Food waste	48.0	6.4	37.6	2.6		4000					5.0		[18]
Meat	42.51	7.03	30.20	13.08	2.74	8100	0.78	1.6	0.018		4.9	75	[19]
Plastics
PVC	36.4	4.43		<0.1	6.3^a	525	70	267	140	3530	4.98	0.19	[7, 20]
Paper
Cardboard	43.0	5.9	44.8	0.3		2000					5.0		[18]
Glossy paper	27.4	3.5	34.5	0.07	0.06	200	139	1415	3.97	1.31	34.5		[19]
Waxed cartons	59.2	9.3	30.1	0.1	0.1
Magazines	32.9	5.0	38.6	0.3		1000					23.3		[18]
Wood
Wood waste	50.4	6.06		0.16	<0.01	419	157	463	50.5	26.9	1.81	10.44	[21]
Yard wastes	41.54	4.79	31.91	0.85	0.30	0.24	6000	2000	—	—	20.37		[19]
Bark	53.7	5.9	39.9	0.43	0.02	0.05	2100	0.04			4.8	52.3	[22]
Others
Shoes	58.8	7.27		1.92	3.20	8610	533	892	6840	243	15.5	2.58	[7]
Leather	54.9	5.1	19.2	14.1	0.8	14,000					5.25	13.3	[23]
Batteries	—	—	—	—	1.55	5300	4900	11,800	1787	830			[24]
Shredder waste	17.3	2.15		0.59	0.33	4890	4630	11,900	22,000	2810	72.8	2.05	[21]
Sewage sludge	53.5	6.4	36.1	2.63	0.12	12,600	3600	3800			28.6	75.6	[22]
Tires	84.39	7.13	2.19	0.24	0.15	12,400	—	—	65	1.53	4.81	0.62	[19]
MSW	39.5	5	26.5	1.5	0.7	3000	2500	2500	300	300	26.5	25	[19]

Table 1.

Ultimate analysis of MSW fractions.

aCited from Ref. [20].

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.

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 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]:

$$\text{2HCl}\left(\text{g}\right)+1/{\text{2O}}_2\left(\text{g}\right)={\text{Cl}}_2\left(\text{g}\right)+{\text{H}}_2\text{O}\left(\text{g}\right)$$E1

$$\text{M}\left(\text{s}\right)+{\text{Cl}}_2\left(\text{g}\right)={\text{MCl}}_2\left(\text{s}\right)$$E2

$${\text{MCl}}_2\left(\text{s}\right)={\text{MCl}}_2\left(\text{g}\right)$$E3

$${\text{2MCl}}_2\left(\text{g}\right)+3/{\text{2O}}_2\left(\text{g}\right)={\text{M}}_2{\text{O}}_3\left(\text{s}\right)+{\text{2Cl}}_2\left(\text{g}\right)$$E4

$${\text{3FeCl}}_2\left(\text{g}\right)+{\text{2O}}_2\left(\text{g}\right)={\text{Fe}}_3{\text{O}}_4\left(\text{s}\right)+{\text{3Cl}}_2\left(\text{g}\right)$$E5

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, 29, 30, 31, 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, 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].

Element	Lowest case scenario (wt.%)	Worst case scenario (wt.%)
K	0.16	0.2
Na	0.04	2.03
Cl	0.01	1.13
S	0.08	0.08
Zn	0.012	0.056
Pb	0.045	0.15

Table 2.

Composition of MSW.

Data was taken from various references [35, 36, 37].

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.

Figure 2.

Influence of KCl on corrosion of iron metal [40].

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 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.

Figure 3.

Corrosion mechanism of pure iron metal.

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 1320 K and typically has high contents of Fe₂O₃ and sulfates and low contents of SiO₂ and Al₂O₃. The powdered ash deposition occurs below 1070 K 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 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 Fe_xO_y. 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.

5. Options to improve MSW treatment efficiency

Waste-to-energy incineration represent an existing technology for efficiently dealing with the most problematic waste issue worldwide. To achieve optimal resource recovery, production of heat and electricity, it require prior separation of recyclables. However, high temperature corrosion, ash deposition, fluctuation in steam temperature, and high operating costs make the treatment of MSW in incinerators a challenging. Several opportunities are available to improve the treatment of wastes in incineration facilities.

5.1 Waste pre-processing

Pre-sorting of waste enables the collection of some waste fractions to be recycled, and those that cannot be recycled be transformed to electricity, as well as production of value added products. Research breakthrough comprises of characterization approaches for high-precision sorting such as the development of max AI recycling robot. The system can work safely alongside people in the system, which allows the Max-AI robotic sorter to be placed into existing material recovery facilities or newly built waste to energy facilities where waste as received from the society is sorted before either recycle, reuse or converted to energy.

5.2 Removal of dissolved pollutants

Quality control measures and pre-treatment processes to remove contaminants can improve the treatment of waste. This encompasses removal of dissolved pollutants from waste such as chlorine, alkali metals and sulfur which causes high temperature corrosion and ash deposition.

To sustainably and efficiently treat MSW in incinerators, minimum chlorine-induced adverse effects should be observed. Therefore, it is crucial to control the dissolved pollutants prior to the burning process such as the removing the alkali chlorides and separating plastic, than attempt to minimize their damage by the chemical dechlorination and catalytic separation. Since all the incineration hindrances are mainly due to the presence of chlorine, much research have focused on understanding the relationships between chlorine in the waste and the HCl formation. An investigation was conducted to determine the amount of HCl that is emitted from PVC and a mixture of waste without the PVC-fraction. It was observed that HCl-emission decreased by 40% to 1.7 mg Cl/g wet MSW [47]. Another authors Delay et al. [48] showed that the decrease in plastics, especially PVC in the MSW result in decreased boiler corrosion as well as decreased heavy metal emissions. At present, the techniques used to separate plastics, are grounded on density. In consideration of its accuracy, three advanced separation processes: the sink-float process, centrifuge and hydrocyclone separation are used, in order to find out how each process would distribute chlorine contained in plastic waste, to the overflow and underflow. It is concluded that with the current sorting technology, the 13% mixed waste plastic go to incinerators, 35% to recycling, and 52% to landfills. With the sink-float separation technology, 47.5% chlorine-poor plastic (<0.5%) goes to incineration plants as alternative fuel, and 16.9% (ca 4.7% Cl content) landfilling.

For the inorganic salt in the waste streams, a water-washing process can be a simple but practical method for inorganic salts removal. Numerous studies on waste and biomass washing have revealed that a large portion of chlorine could be released by elution tests. Jensen et al. [49] reported that approximately 90% of the salts can be removed from biomass char, within 20 min by water washing. Chen and Pagano [50], observed that application of a high temperature (93.7°C) leaching technique on a high chlorine coal (0.5 wt.%) result in chlorine decrease to about 0.2 wt.%. In MSW approximately 85% of easily water-soluble chlorine was leached out in the first washing procedure by distilled water [51]. The limitation of the water extraction method is on increasing the total costs of the operations as well as disposal of the effluent.

5.3 Optimisation of combustion environment

Optimization of combustion conditions in incineration plant had recently emerged as a method to reduce the corrosion impacts in waste to energy plants. Among the key methods, installations of the Segher Boiler Prisms in incineration facilities [47] has been utilized as a measure reduce corrosion in the boiler. The technology consist of a prism-shaped dynamic secondary air mixer, that is inserted at the transition of the combustion chamber and is cooled by flowing water, lined by refractory materials, and has the natural circulation system. The prism splits the flue gas into two channels where each is supplied by secondary air injection. Prism ensures homogenous injection of secondary air via multiple nozzles that are on the prism sides and boiler walls. This supply ensures that there is high turbulence and mixing of waste with excess air which prevents the deposition of ash and also facilitates sulphation process to occur [52], which in turn reduces the corrosion impact [47]. In addition, the prism results in a uniform distribution of flue gas speed, temperature and oxygen, thereby preventing the creation of hot spots within the boiler [32]. Moreover, the prism removes heat from the combustion zone, and reduces the temperature, which then reduces the volatilization of alkali chlorides, leading to generation of less chemically corrosive deposits [47, 52].

6. Future perspectives and knowledge gaps

The present chapter aims for an investigation, understanding, and analysis of the factors that lead to unsustainable treatment of MSW in incinerators. Several research directions have been identified that may help in the improvement of waste treatment in the waste to energy incinerators.

High temperature corrosion is a problematic issue that is hindering the sustainable and efficient treatment of waste in incinerators. The general theory of the corrosion process is the active oxidation. Most researchers studied the global corrosion reaction process. However, studies on the elementary reactions and their reaction rate constant which actually shows how the corrosion process occurs are scarce and this can be achieved via kinetic modeling of corrosion on various alloys as well as various operating conditions which needs to be validated by full-scale experimental investigations. Nonetheless, the operation conditions in full scale set up are difficult to control. In light of that, lab scale experiment can be used to validate the modeling. This may help plant operators to select the operating conditions that result in low corrosion rate as well as on alloy which can be used as superheater thereby improve the treatment efficiency of waste.

More research is needed on the optimization of the incineration conditions through the use of various technologies including Ecotube and Segher boiler prism which reduces corrosion and ash deposition by ensuring high turbulence and optimal mixing within the boiler but studies showing these technologies are rare.

7. Conclusions

Waste-to-energy incinerators have been generally accepted as an environmentally preferred method for waste management. However, sustainable and efficient treatment of waste in incinerators is hindered by high temperature corrosion and ash deposition. This studies evaluated the potential for improving the efficient treatment of waste in incinerators. It is found that optimizing the operating conditions during incineration reduces the hindrance problem of high temperature corrosion and ash deposition. The operating conditions can be optimized by conducting a kinetic modeling which identifies the conditions that leads to reduced corrosion rate. Also, use of new technologies such the ecotube and sergher boiler prism which ensures high turbulence and optimal mixing within the boiler reduces the corrosion ad ash deposition problem, thereby improving the incineration of waste. Moreover, sorting of waste and related feedstocks improves the treatment of waste in incinerators as chlorine containing waste fractions may have been removed and this can be achieved through the use of technologies such as the max Al robotic sorter. Furthermore, to efficiently treat MSW in incinerators, chlorine load prior to waste incineration should be reduced by washing alkali chlorides and separating plastic from the waste as received. Figure 4 summarizes the hindrances and options to improve MSW treatment efficiency during incineration.

Figure 4.

Framework for options to improve MSW treatment efficiency.

Acknowledgments

The authors are thankful to the IntechOpen editor for the invitation to contribute this chapter of the book. Also, the author is thankful to Prof. Gwenzi affiliated at University of Zimbabwe for the insightful guidance during the writing of this chapter.

Conflict of interest

The author declare that there is no conflict of interest.