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Article

Screening of Potential Additives for Alleviating Slagging and Fouling during MSW Incineration: Thermodynamic Analysis and Experimental Evaluation

by
Gang Chen
1,
Shunshun Kang
2,3,4,
Kun Zhao
2,3,4,*,
Anqing Zheng
2,3,4,* and
Zengli Zhao
2,3,4
1
China Southern Power Grid Technology Co., Ltd., Guangzhou 510170, China
2
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China
3
CAS Key Laboratory of Renewable Energy, Guangzhou 510640, China
4
Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China
*
Authors to whom correspondence should be addressed.
Atmosphere 2022, 13(8), 1163; https://doi.org/10.3390/atmos13081163
Submission received: 14 June 2022 / Revised: 28 June 2022 / Accepted: 30 June 2022 / Published: 22 July 2022
(This article belongs to the Special Issue Syngas Production by Chemical Looping Gasification)
Browse Figures
Versions Notes

Abstract

:
The formation of slagging and fouling during municipal solid waste (MSW) incineration not only significantly affects heat transfer, but also results in shortened operating cycles. In order to solve the issues, the effect of different additives on the migration and transformation patterns of alkali/alkaline earth metals (AAEM) and chlorine during MSW incineration is screened based on the Gibbs energy minimization method. The effect of potential additives on the ash fusion temperature and combustion reactivity of MSW char is subsequently verified and evaluated by experimental methods. The thermodynamic equilibrium analysis shows that Al(NO3)3, Ca(NO3)2, and Mg(NO3)2 have great potential to increase the ash fusion temperature. The experimental investigation confirms that the addition of Al(NO3)3, Ca(NO3)2, and Mg(NO3)2 significantly increases the ash fusion temperature. The order of increasing the ash fusion temperature by different additives is Mg(NO3)2 > Ca(NO3)2 > Al(NO3)3. The addition of Mg(NO3)2 significantly increased the initial deformation temperature, softening temperature, hemispheric temperature, and flow temperature of ash from 1180, 1190, 1200, and 1240 °C to 1220, 1230, 1240, and 1260 °C, respectively. The addition of Cu(NO3)2, Fe(NO3)3, and KMnO4 significantly decreases the temperature at the maximum weight loss rate of MSW char, while increasing the maximum weight loss rate. Additionally, Cu(NO3)2 shows the best performance in improving the combustion reactivity of MSW char. The addition of Cu(NO3)2 evidently increases the maximum weight loss rate from 0.49 to 0.54% °C−1. Therefore, it is concluded that Mg(NO3)2 and Cu(NO3)2 are supposed to be the most potential candidates for efficient additives. This study presents an efficient and economical method to screen potential additives for alleviating slagging and fouling during MSW incineration.

1. Introduction

With the rapid development of China’s economy and people’s living standards, the amount of municipal solid waste (MSW) generated is also increasing rapidly [1,2]. Landfill disposal of MSW not only takes up a lot of land, but also causes serious pollution of soil, groundwater, and air [3]. Incineration, as an efficient thermochemical conversion technique, can realize the reduction, harmless and resourceful utilization of MSW, which is important to achieve the sustainable development of society [4,5,6]. However, MSW usually contains large numbers of alkali/alkaline earth metals (AAEM), chlorine, silicon, etc., which leads to the low ash fusion temperature of MSW via the formation of low melting compounds or eutectics during incineration [7,8]. When the incineration temperature of MSW is higher than the ash fusion temperature, the ash will reach the molten or semi-molten state in the high-temperature region, and then it will easily adhere to the heat-transfer surfaces and then continue to capture the molten or semi-molten ash and unburned coke [7,9]. The combination of alkali metals and other elements (e.g., chlorine) will be released as gas-phase products and aerosols, which will condense on the surface of solid particles to form agglomerates with a sticky molten film, and then adhere to the heat-transfer surfaces [9,10,11,12]. The formation of slagging and fouling not only affects heat transfer, but also leads to shortened operating cycles. It is generally believed that slagging and fouling are, respectively, formed on the radiative heat-exchange surface and the convective heat-transfer surface [13].
The main methods currently used to alleviate slagging and fouling include pretreatment of MSW by water washing, optimization of operating parameters of incinerators, blending different feedstocks, and screening of potential additives [9,14]. Among them, screening of potential additives is an efficient and economical method to minimize slagging and fouling propensity during MSW incineration [15,16]. The additives are usually used to alter the fusion properties of ash. The commonly used additives for alleviating slagging and fouling include aluminosilicates, kaolin, SiO2, Al2O3, TiO2, (NH4)2SO4, Al2(SO4)3, Fe2(SO4)3, (NH4)3PO4, H3PO4, CaCO3, CaO, zeolites, etc. Some additives can promote the evaporation of AAEM species to reduce the slagging risk of bottom ash. However, these AAEM species can easily condense on the heat-transfer surfaces to further cause fouling and corrosion [14]. More effective additives are used to trap the AAEM species and form more stable species with high melting points [14]. In addition, enhancing the reactivity of MSW char with additives not only helps alleviate the slagging and fouling, but also improves the combustion efficiency during MSW incineration [17]. Therefore, an in-depth investigation into the effect of different potential additives on the migration and transformation of the main elements in ash (e.g., alkali metals) and the ash fusion temperature during MSW incineration is the key to solving the issues of slagging and fouling. Since the MSW incineration system is a complex multiphase reaction system at high temperatures, it is difficult to investigate the formation mechanism of slagging and fouling by traditional experimental methods, and thermodynamic equilibrium analysis of MSW incineration can provide some reference for experimental investigation. There are many studies focused on the migration and transformation of alkali metals during the combustion of coal and biomass [18,19,20]. Previous studies showed that the reaction temperature, excess air coefficient, and Cl content have a great influence on the migration and transformation of alkali metals during MSW incineration [21,22,23]. However, there are few studies focused on the effect of additives on the migration and transformation of alkali metals and ash fusion temperature during MSW incineration by using thermodynamic analysis combing with the experimental investigation. Therefore, in this study, the effect of incineration temperature and different additives on the migration and transformation patterns of AAEM and chlorine during MSW incineration is investigated based on the Gibbs energy minimization method. The effect of potential additives on the ash fusion temperature is subsequently verified. Moreover, the effect of potential additives on the combustion reactivity of MSW-derived char is evaluated. The potential efficient additives are thus proposed.

2. Experimental Section

2.1. Thermodynamic Equilibrium Analysis

The MSW samples are obtained from Guangzhou. The physicochemical properties of MSW samples are provided in Table 1 and Table 2. The moisture content of MSW is 38.43%. Thermodynamic equilibrium analysis using the Gibbs energy minimization method was carried out to explore the migration and transformation of typical ash components in MSW samples at different reaction temperatures during the MSW incineration process, especially for AAEM, chlorine, and silicon. The detailed method for thermodynamic equilibrium analysis is described in the previous studies [24,25]. Moreover, the effect of potential additives on the migration and transformation of AAEM, chlorine, and other elements was systematically studied at a reaction temperature of 900 °C and atmospheric pressure by thermodynamic equilibrium analysis.

2.2. Screening of Potential Fouling Preventatives

The ash samples were prepared from the MSW samples according to the national standard GB/T 28731-2012. The prepared ash samples were mixed with the potential additives obtained by thermodynamic screening, and the effect of the potential additives on the ash fusion temperature was investigated, the potential additives of Al2(NO3)3, Ba(NO3)2, NH4 H2PO4, Ca(NO3)2, and Mg(NO3)2 were used as fouling preventatives, which were dissolved in deionized water to prepare a solution. The ash samples are subsequently added to the solutions under vigorous stirring to ensure the ash samples and the additives mixed evenly, and the solutions were dried at 105 °C for 24 h. The ash fusion temperature was measured by an automatic ash fusion temperature tester according to the Chinese standard GB/T219-2008. By comparing the changes in the cone shape, the fusion temperature of the ash samples, namely the initial deformation temperature (DT), the softening temperature (ST), the hemispheric temperature (HT), and the flow temperature (FT), were obtained.

2.3. Screening of Potential Additive for Enhancing the Combustion Reactivity of MSW Char

The MSW char was obtained via the pyrolysis of MSW samples in a closed crucible at 900 °C for 7 min. The components of MSW are listed in Table 3. The potential additives (KMnO4, Cu(NO3)2, Fe(NO3)3, and Ba(NO3)2) were dissolved in deionized water to prepare a solution. The MSW char was impregnated with the solution under stirring to ensure they mixed evenly and then dried at 105 °C for 24 h. The combustion reactivity was tested by a thermogravimetric analyzer (STA40PC, Netzsch, Germany) from 105 °C and 900 °C with a heating rate of 10 °C/min and an air flowrate of 100 mL/min.

3. Results and Discussion

3.1. Effect of Incineration Temperature and Different Additives on the Migration and Transformation of Major Elements in Ash by Using Thermodynamic Equilibrium Analysis

3.1.1. Effect of Incineration Temperature on the Migration and Transformation of Major Elements in Ash by Using Thermodynamic Equilibrium Analysis

The effect of incineration temperature on the migration and transformation of the major elements in ash during MSW incineration was investigated by the thermodynamic equilibrium analysis. As shown in Figure 1a, when the temperature exceeds 750 °C, most of the sodium starts to convert to gaseous sodium chloride, and a small amount of the sodium reacts with silicon and aluminum to generate Na2SiO3, NaAlSiO4, and other compounds. The formation of Na2SiO3 and NaAlSiO4 reach their maximum value at 850 °C and 1100 °C, respectively. The melting point of Na2SiO3 is approximately 1088 °C [26]. A large amount of Na2SiO3 generated during MSW incineration could reduce the ash fusion temperature and aggravate the slagging and fouling. At the same time, gaseous NaCl could react with the metals in the heat-transfer surfaces, resulting in the serious corrosion of heat-transfer surfaces. As can be seen from Figure 1b, the distribution of potassium is similar to that of sodium. At 1000 °C, most of the potassium exists in the form of gaseous potassium chloride, which will also cause the fouling and corrosion of the heat-transfer surfaces. Its corrosion principle is similar to that of sodium chloride. It can be seen from Figure 1c that the distribution of calcium is relatively complex. When the temperature is lower than 500 °C, the calcium mainly exists in the form of CaCO3 and CaSiO3. As the temperature continues to rise, the CaCO3 starts to decompose and most of the calcium is present mainly in the form of silicates. The formation of CaSiO3 could reduce the ash fusion temperature, while refractory minerals such as CaAl2Si3O10(OH)2 decomposes below 900 °C, which further reduces the ash fusion temperature [27]. The effect of incineration temperature on the migration and transformation of magnesium is given in Figure 1d, and it is found that, in the low-temperature region, magnesium always coexists with calcium, and more magnesium silicate and magnesium oxide are formed as the temperature increases. As shown in Figure 1e, chlorine is always preferred to form highly volatile chlorides with alkali metals. Additionally, a small amount of HCl will be generated at the same time, which is easy to cause low-temperature corrosion. Figure 1f shows the effect of incineration temperature on the migration and transformation of silicon. The silicon is prone to react with sodium, potassium, and other elements to form low-melting eutectic mixtures at high temperatures, resulting in the slagging and fouling. Below 1000 °C, silicon mainly exists in the form of CaAl2SiO10(OH)2, CaSiO3, and Ca3Fe2Si3O12.

3.1.2. Effect of Al(NO3)3 Addition on the Migration and Transformation of Major Elements in Ash by Using Thermodynamic Equilibrium Analysis

It is evident that the addition of Al(NO3)3 exerts a significant effect on the migration and transformation of the main elements in ash. As shown in Figure 2a, when the amount of Al(NO3)3 addition increases from 0 to 0.1 kmol, the content of Na2SiO3 decreases drastically. As the amount of Al(NO3)3 addition continues to increase, the content of gaseous NaCl gradually decreases, whereas the content of NaAlO2 increases rapidly. The melting point of NaAlO2 is approximately 1650 °C, which can effectively increase the ash fusion temperature [28]. It can be seen from Figure 2b that the content of 3KCl·MgCl2 changes slightly, and the addition amount of Al(NO3)3 does not affect the distribution of potassium. Figure 2c and d show that, when the amount of Al(NO3)3 addition increases, the content of CaAl2Si3O10(OH)2 first increases obviously and then decreases steadily. When the amount of Al(NO3)3 addition increases from 0 to 0.5 kmol, the contents of CaO·Al2O3, CaO·2Al2O3, and MgO·2Al2O3 with high melting point first increase rapidly, and then their contents tend to keep stable. As shown in Figure 2e, the change in the distribution of chlorine is mainly related to the transformation of part of gaseous NaCl into HCl. It can be seen from Figure 2f that, when the amount of Al(NO3)3 addition is between 0.1–0.4 kmol, the silicon mainly exists in the form of CaAl2Si3O10 (OH)2. As the amount of Al(NO3)3 addition further increases, the content of Al2O3·SiO2(D) increases continuously since CaAl2Si3O10(OH)2 is converted into Al2O3·SiO2(D). It can be inferred that the addition of Al(NO3)3 has great potential for increasing the ash fusion temperature. The results are in accordance with the previous studies, which demonstrated that aluminum-based additives could improve the fusion temperature of ash particles to suppress the formation of low-temperature eutectics.

3.1.3. Effect of Mg(NO3)2 Addition on the Migration and Transformation of Major Elements in Ash by Using Thermodynamic Equilibrium Analysis

The effect of the amount of Mg(NO3)2 addition on the migration and transformation of sodium, potassium, calcium, magnesium, chloride, and silicon is illustrated in Figure 3a–f, respectively. It is observed that the addition of Mg(NO3)2 has a relatively small impact on the migration and transformation of sodium, potassium, and chlorine. On the contrary, the addition of Mg(NO3)2 exerts a significant effect on the migration and transformation of calcium. As the amount of Mg(NO3)2 addition increases, the content of CaO·MgO·SiO2 with a high melting point increases gradually, whereas the content of CaSiO3 decreases evidently. After the addition of Mg(NO3)2, the content of MgO improves significantly, since Mg(NO3) is mainly decomposed into MgO under the thermal action, and only a small amount of Mg(NO3)2 reacts with SiO2 to generate Mg2SiO4. The addition of Mg(NO3)2 also markedly affects the migration and transformation of silicon. As the amount of Mg(NO3)2 addition increases, the contents of CaAl2Si3O10(OH)2, CaSiO3, and Ca3Si2O7 decline progressively, whereas the content of CaO·MgO·SiO2 increases considerably. The addition of Mg(NO3)2 significantly promotes the formation of new compounds with high melting point, suggesting that the addition of Mg(NO3)2 could be capable of enhancing the ash fusion temperature.

3.1.4. Effect of Ca(NO3)2 Addition on the Migration and Transformation of Major Elements in Ash by Using Thermodynamic Equilibrium Analysis

The effect of the amount of Ca(NO3)2 addition on the migration and transformation of sodium, potassium, calcium, magnesium, chloride, and silicon is drawn in Figure 4a–f, respectively. Similar to Mg(NO3)2, Ca(NO3)2 imposes an obvious effect on the migration and transformation of calcium, magnesium, and silicon. The migration and transformation of calcium can be divided into two stages. When the amount of Ca(NO3)2 addition is less than 0.5 kmol, Ca(NO3)2 mainly reacts with silicate to form Ca3SiO5. When the amount of Ca(NO3)2 addition exceeds 0.5 kmol, the content of Ca3SiO5 drops quickly, and the Ca(NO 3)2 is mainly decomposed into CaO. After the addition of Ca(NO3)2, the dominant compounds of Mg are changed to MgO and CaO·MgO. Both Ca3SiO5, CaO, MgO, and CaO·MgO are high melting point compounds, implying that the addition of Ca(NO3)2 is also able to promote the ash fusion temperature.

3.1.5. Effect of Ba(NO3)2 Addition on the Migration and Transformation of Major Elements in Ash by Using Thermodynamic Equilibrium Analysis

The effect of the amount of Ba(NO3)2 addition on the migration and transformation of sodium, potassium, calcium, magnesium, chloride, and silicon is given in Figure 5a–f, respectively. The effect of the addition of Ba(NO3)2 on the migration and transformation of sodium, potassium, and chlorine is similar to that of Mg(NO3)2 and Ca(NO3)2. After the addition of Ba(NO3)2, the major forms of sodium, potassium, and chlorine are still gaseous NaCl and 3KCl·MgCl2, while the forms of calcium and magnesium are altered to CaO and MgO. It can be seen from Figure 5f that silicate ions are preferentially combined with barium ions to form Ba2SiO4 after the addition of Ba(NO3)2. As the amount of Ba(NO3)2 addition increases from 0 to 0.4 kmol, the contents of Ba2SiO4, CaO, and MgO increase considerably. When the addition amount of Ba(NO3)2 exceeds 0.5 kmol, the contents of Ba2SiO4, CaO, and MgO tend to remain stable.

3.1.6. Effect of NH4HPO4 Addition on the Migration and Transformation of Major Elements in Ash by Using Thermodynamic Equilibrium Analysis

The effect of the amount of NH4HPO4 addition on the migration and transformation of sodium, potassium, calcium, magnesium, chloride, and silicon is provided in Figure 6a–f, respectively. It is found that the addition of NH4HPO4 exerts significant impacts on the migration and transformation of alkali and alkaline earth metals, which are always preferentially combined with phosphates to form various metal phosphates. As shown in Figure 6, without the addition of NH4HPO4, sodium and potassium primarily exist in the form of NaCl and 3KCl·MgCl2. The sodium and potassium are, respectively, transformed into NaPO3 and KPO3 after the addition of NH4HPO4. The migration and transformation of calcium and magnesium is similar to that of sodium and potassium. The calcium and magnesium react preferentially with phosphates to form Ca(OH)2∙Ca3(PO4)2, Mg2P2O7, and Mg3(PO4)2. Previous studies showed that phosphorus-based additives can effectively retain the AAEM in bottom ash by forming highly thermally stable AAEM phosphates. After the addition of NH4HPO4, chlorine mainly exists in the form of gaseous HCl, since the combination of phosphate ions and sodium ions to form NaPO3 suppresses the formation of NaCl. At the same time, the form of silicon changes from CaAl2Si3O10(OH)2 to SiO2. The melting point of Mg2P2O7 is as high as 1380 °C [29]. However, KPO3 and NaPO3 exhibit very low melting points, which could promote the occurrence of slagging and fouling. Therefore, it is inferred that the addition of NH4HPO4 could reduce the ash fusion temperature.

3.1.7. Effect of Cu(NO3)2 Addition on the Migration and Transformation of Major Elements in Ash by Using Thermodynamic Equilibrium Analysis

The effect of the amount of Cu(NO3)2 addition on the migration and transformation of sodium, potassium, calcium, magnesium, chloride, and silicon is depicted in Figure 7a–f, respectively. The addition of Cu(NO3)2 shows the same effect as that of Ba(NO3)2. The migration and transformation of sodium, potassium, and chlorine do not change significantly after the addition of Cu(NO3)2. The main forms of sodium, potassium, and chlorine are gaseous NaCl and 3KCl·MgCl2. The addition of Cu(NO3)2 alters the main forms of calcium and magnesium. As shown in Figure 7c,d, the addition of Cu(NO3)2 favors the formation of MgO and CaO, which could be attributed to the competitive reactions of copper, calcium, and magnesium with silicon. It is observed that Cu(NO3)2 can react with silicate ions to generate large amounts of, Cu6Si6O18 ·6H2O, which is thermodynamic preferred. The Cu6Si6O18 ·6H2O thus becomes the main phase of silicon.

3.1.8. Effect of KMnO4 Addition on the Migration and Transformation of Major Elements in Ash by Using Thermodynamic Equilibrium Analysis

The effect of the amount of KMnO4 addition on the migration and transformation of sodium, potassium, calcium, magnesium, chloride, and silicon is shown in Figure 8a–f, respectively. The potassium in KMnO4 and the sodium in ash will compete with other elements during MSW incineration. As the amount of KMnO4 addition increases, potassium will dominate the competitive reactions. As shown in Figure 8, when the amount of KMnO4 addition is between 0 and 1.0 Kmol, KMnO4 mainly reacts with chlorine to form 3KCl·MgCl2, which becomes the major form of chlorine and magnesium. At the same time, sodium is converted from gaseous NaCl to Na2SiO3, and potassium mainly exists in the form of K2CO3 and K2SiO3. When the amount of KMnO4 addition exceeds 1.0 kmol, sodium and potassium primarily exist in the form of NaMnO4 and KOH, respectively. It is worth noting that NaMnO4 is not stable at high temperatures, and is prone to undergo decomposition reactions to generate oxygen, which is beneficial for the combustion of char during MSW incineration. The addition of KMnO4 can promote the reaction between potassium and silicate ions to form K2SiO3. However, the melting point of K2SiO3 is 976 °C [30]. Hence, the addition of KMnO4 is not conducive to improving the ash fusion temperature, but it has the potential to improve the combustion behavior of MSW as a strong oxidant.

3.2. Influence of the Potential Additives on the Ash Fusion Temperature

Several representative potential additives, such as Al(NO3)3, Ba(NO3)2, Ca(NO3)2, Mg(NO3)2, and NH4H2PO4 are selected and added to the ash sample for verifying the speculation obtained from the thermodynamic equilibrium analysis mentioned above. The effect of various potential additives on the ash fusion temperature is shown in Figure 9. The different additives have very distinct influences on the ash fusion temperature. After the addition of NH4H2PO4, the initial deformation temperature, softening temperature, and hemispherical temperature of the ash all decrease by approximately 20 °C. However, the flow temperature does not exceed the original flow temperature of the ash. According to the thermodynamic results in Figure 6, it is speculated that, after the addition of NH4H2PO4, large amounts of metaphosphates such as KPO3 and NaPO3 with low melting points are formed, thus leading to the decreases in the ash fusion temperature. However, after the addition of Ba(NO3)2, the initial deformation temperature, softening temperature, and hemispherical temperature change slightly, except for the flow temperature. The decrease in flow temperature of ash caused by the addition of Ba(NO3)2 may be due to the formation of Ba2SiO4. Therefore, NH4H2PO4 and Ba(NO3)2 are not suitable as additives to alleviate the problems of slagging and fouling during MSW incineration. On the contrary, the addition of Al(NO3)3, Ca(NO3)2, and Mg(NO3)2 significantly improves the ash fusion temperature. The rank order in the improvement of ash fusion temperature is Mg(NO3)2 > Ca(NO3)2 > Al(NO3)3, and Mg(NO3)2 exhibits the best performance. The addition of Mg(NO3)2 increases the deformation temperature, softening temperature, and hemispherical temperature of the ash by around 40 °C, reaching 1220 °C, 1230 °C, and 1240 °C, respectively. Al(NO3)3 increases the deformation temperature, softening temperature, and hemispheric temperature by about 10 °C. It is thus concluded that Mg(NO3)2, Ca(NO3)2, and Al(NO3)3 are efficient additives to alleviate the problems of slagging and fouling during MSW incineration.

3.3. Effect of Potential Additives on the Combustion Reactivity of MSW Char

The effect of potential additives on the derivative thermogravimetric (DTG) curves for the combustion of MSW char is given in Figure 10. There are two main weight loss peaks in the DTG curves for the combustion of MSW char. The first weight loss peak appears between 300–550 °C, corresponding to the combustion of MSW char. It is found that the addition of Cu(NO3)2 and KMnO4 shifts the first peak temperature towards the lower temperature region while improving the maximum weight loss rate, indicating that these additives can serve as catalysts to accelerate the combustion reaction of MSW char. At the same time, it is observed that the addition of Ba(NO3)2 not only shifts the first peak temperature toward a higher temperature, but also reduces the maximum weight loss rate, suggesting that the addition of Ba(NO3)2 is not conducive to the combustion reaction. The second weight loss peak appears between 600–700 °C, which is mainly ascribed to the decomposition of minerals in MSW char, such as CaCO3.
To further quantitative evaluation of the effect of different additives on the combustion reactivity of MSW char, the ignition temperature, the temperature at maximum weight loss rate, and the maximum weight loss rate are used in this experiment to describe the combustion reactivity of MSW char. The ignition temperature is determined by the intersection method, that is, the point corresponding to the temperature at maximum weight loss rate on the TG curve is drawn as a tangent, and it intersects with the horizontal line where the weight loss begins, and the temperature corresponding to this point is the ignition temperature [31]. Table 4 shows the combustion characteristic parameters of MSW char. After the addition of Cu(NO3)2, Fe(NO3)3, and KMnO4, the ignition temperatures of MSW char slightly drop. The results could be attributed to the catalytic effect of these additives on the oxidation reaction of MSW char particles [32]. The temperature at the maximum mass loss rate of MSW char is 468.1 °C. The addition of Cu(NO3)2, Fe(NO3)3, and KMnO4 significantly reduces the temperature at the maximum mass loss rate of MSW char to 452.7, 454.5, and 459.7 °C, respectively. Simultaneously, the addition of Cu(NO3)2, Fe(NO3)3, and KMnO4 increase the maximum mass loss rate. Compared with the maximum weight loss rate (0.49%·°C−1) of MSW char without additives, the addition of Cu(NO3)2 obviously improves the maximum weight loss rate to 0.54% °C−1. These results confirm that the addition of Cu(NO3)2, Fe(NO3)3, and KMnO4 can promote the combustion reactivity of MSW char. However, the addition of Ba(NO3)2 increases the ignition temperature and the temperature at maximum weight loss rate while decreasing the maximum mass loss rate, implying that Ba(NO3)2 can inhibit the combustion reaction of MSW char. Considering these results from both thermodynamic equilibrium analysis and experimental investigation, Mg(NO3)2, Ca(NO3)2, Al(NO3)3 and Cu(NO3)2 are supposed to be the most potential candidates for efficient additives.

4. Conclusions

The thermodynamic equilibrium analysis shows that Al(NO3)3, Ca(NO3)2, and Mg(NO3)2 have great potential for increasing the ash fusion temperature. The experimental investigation verifies that the addition of Al(NO3)3, Ca(NO3)2, and Mg(NO3)2 significantly improves the ash fusion temperature. Additionally, Mg(NO3)2 provides the highest improvement in the ash fusion temperature. The thermogravimetric analysis shows that the addition of Cu(NO3)2, Fe(NO3)3, and KMnO4 significantly reduces the temperature at the maximum mass loss rate of MSW char while increasing the maximum mass loss rate. Additionally, Cu(NO3)2 exhibits the best performance in enhancing the combustion reactivity of MSW char. It is thus concluded that Mg(NO3)2 and Cu(NO3)2 are supposed to be the most potential candidates for efficient additives from the comprehensive consideration of thermodynamic equilibrium analysis and experimental investigation. This study demonstrates that thermodynamic equilibrium analysis is an efficient and economical manner to screen the potential additives for alleviating slagging and fouling during MSW incineration.

Author Contributions

Conceptualization, K.Z. and A.Z.; methodology, G.C. and S.K.; validation, G.C. and S.K.; investigation, G.C. and S.K.; resources, G.C. and S.K.; data curation, G.C. and S.K.; writing—original draft preparation, G.C., S.K., K.Z., and A.Z.; writing—review and editing, K.Z. and A.Z.; supervision, K.Z., A.Z. and Z.Z.; project administration, K.Z. and A.Z.; funding acquisition, G.C. and A.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Grants 51876205, 51876208, and 51776209) and Youth Innovation Promotion Association, CAS (2019341).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors acknowledge theNational Natural Science Foundation of China (Grants 51876205, 51876208 and 51776209) and Youth Innovation Promotion Association, CAS (2019341), for their financial supports of this work.

Conflicts of Interest

The authors declare no conflict of interest.

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Atmosphere 13 01163 g001 550
Figure 1. Effect of incineration temperature on the migration and transformation of the major elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
Figure 1. Effect of incineration temperature on the migration and transformation of the major elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
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Figure 2. Effect of the amount of Al(NO3)3 addition on the migration and transformation of main elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
Figure 2. Effect of the amount of Al(NO3)3 addition on the migration and transformation of main elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
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Atmosphere 13 01163 g003 550
Figure 3. Effect of the amount of Mg(NO3)2 addition on the migration and transformation of main elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
Figure 3. Effect of the amount of Mg(NO3)2 addition on the migration and transformation of main elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
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Atmosphere 13 01163 g004 550
Figure 4. Effect of the amount of Ca(NO3)2 addition on the migration and transformation of main elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
Figure 4. Effect of the amount of Ca(NO3)2 addition on the migration and transformation of main elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
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Figure 5. Effect of the amount of Ba(NO3)2 addition on the migration and transformation of main elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
Figure 5. Effect of the amount of Ba(NO3)2 addition on the migration and transformation of main elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
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Figure 6. Effect of the amount of NH4HPO4 addition on the migration and transformation of main elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
Figure 6. Effect of the amount of NH4HPO4 addition on the migration and transformation of main elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
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Figure 7. Effect of the amount of Cu(NO3)2 addition on the migration and transformation of main elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
Figure 7. Effect of the amount of Cu(NO3)2 addition on the migration and transformation of main elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
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Figure 8. Effect of the amount of KMnO4 addition on the migration and transformation of the main elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
Figure 8. Effect of the amount of KMnO4 addition on the migration and transformation of the main elements in ash. (a): Na, (b): K, (c): Ca, (d): Mg, (e): Cl, (f): Si. The asterisk in the chemical formula shows that the molecules or species are attached with each other as independent species, similarly hereinafter.
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Figure 9. Effect of various potential additives on the ash fusion temperature, DT: the initial deformation temperature, ST: softening temperature, HT: hemispherical temperature, FT: flow temperature.
Figure 9. Effect of various potential additives on the ash fusion temperature, DT: the initial deformation temperature, ST: softening temperature, HT: hemispherical temperature, FT: flow temperature.
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Figure 10. Effect of potential additives on the combustion reactivity of MSW char.
Figure 10. Effect of potential additives on the combustion reactivity of MSW char.
Atmosphere 13 01163 g010
Table
Table 1. Elemental analysis of MSW (dry basis).
Table 1. Elemental analysis of MSW (dry basis).
ElementCHONSAsh
Content/%44.436.7125.470.790.1422.45
Table
Table 2. Ash composition of MSW.
Table 2. Ash composition of MSW.
CompoundFe2O3SiO2CaOAl2O3Na2OMgOK2OClSO3
Content/%30.8329.4820.305.744.032.270.294.561.51
Table
Table 3. The components of MSW.
Table 3. The components of MSW.
ComponentPaperPlasticLeather RubberClothWood and GrassKitchen WasteFoam
Content/%12.2726.385.2312.372.88391.87
Table
Table 4. Effect of potential additives on the combustion characteristic parameters of MSW char.
Table 4. Effect of potential additives on the combustion characteristic parameters of MSW char.
AdditivesAdded AmountCombustion Reactivity
Ignition Temperature/°CTemperature at Maximum Weight Loss Rate/°CMaximum Weight Loss Rate/(% °C−1)
Cu(NO3)21%399.0452.70.54
KMnO41%399.8459.70.51
Ba(NO3)21%410.5471.90.48
Fe(NO3)31%401.4454.50.50
Control-403.2468.10.49
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Chen, G.; Kang, S.; Zhao, K.; Zheng, A.; Zhao, Z. Screening of Potential Additives for Alleviating Slagging and Fouling during MSW Incineration: Thermodynamic Analysis and Experimental Evaluation. Atmosphere 2022, 13, 1163. https://doi.org/10.3390/atmos13081163

AMA Style

Chen G, Kang S, Zhao K, Zheng A, Zhao Z. Screening of Potential Additives for Alleviating Slagging and Fouling during MSW Incineration: Thermodynamic Analysis and Experimental Evaluation. Atmosphere. 2022; 13(8):1163. https://doi.org/10.3390/atmos13081163

Chicago/Turabian Style

Chen, Gang, Shunshun Kang, Kun Zhao, Anqing Zheng, and Zengli Zhao. 2022. "Screening of Potential Additives for Alleviating Slagging and Fouling during MSW Incineration: Thermodynamic Analysis and Experimental Evaluation" Atmosphere 13, no. 8: 1163. https://doi.org/10.3390/atmos13081163

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