Optimizing Acid Gas Emission Control in Waste Incineration Through Calcium Hydroxide Injection

South Africa’s environmental regulations are becoming increasingly stringent, particularly when it comes to controlling emissions of harmful acid gases such as SO₂, HCl, HF, NO_x, SO₃, Hg, and particulate matter [1]. This evolving regulatory landscape reflects a global shift toward stricter environmental standards driven by growing awareness of climate change, public health concerns, and the need for sustainable development. At the international level, agreements such as the Paris Agreement and the World Health Organization’s air quality guidelines have set ambitious goals for cutting pollutants that contribute to global warming and respiratory illnesses. In parallel, regulatory bodies like the European Union have strengthened emission controls through directives like the Industrial Emissions Directive (IED), which calls for advanced abatement technologies to curb industrial pollution [2]. Similarly, the United States Environmental Protection Agency (EPA) continues to update its National Ambient Air Quality Standards (NAAQS) to reduce pollutants like SO_x and NO_x, which are significant contributors to acid rain and smog [3]. Despite evolving regulations, many industries, including municipal solid waste incineration (MSWI), continue to face challenges in achieving compliance with the increasingly stringent standards. These difficulties are primarily due to the limitations of current emission control technologies. This situation presents an opportunity for innovative solutions to address the growing environmental concerns associated with incineration processes.

These tightening global and local regulations have sparked a surge of interest in innovative pollution control technologies, such as Dry Sorbent Injection (DSI). DSI is increasingly favoured for its relatively low upfront costs, its compact and adaptable equipment design [4]. DSI is particularly relevant in industries like MSWI, where incineration is still a preferred treatment method despite being highly controversial [5]. The ongoing debate over incineration encompasses a wide range of perspectives, with stakeholders including waste producers, facility operators, regulatory bodies, environmental activists, and local communities. These groups are often at odds, balancing the practical need for waste incineration against concerns about its potential threats to human health and the environment [6]. As air quality regulations grow stricter worldwide, industries are under increasing pressure to adopt cutting-edge, sustainable technologies, making it clear that the conversation around incineration and emissions control is far from over. To meet these challenges, this study proposes the use of Calcium Hydroxide (Ca(OH)₂) as a viable solution for reducing harmful acid gas emissions from MSWIs. The novelty of this research lies in its focus on the application of Ca(OH)₂ injections at higher flue gas temperatures. The aim is to determine whether Ca(OH)₂ can effectively reduce emissions like SO₂, NO_x, HCl, and HF and help facilities achieve compliance with the increasingly stringent environmental standards.

To mitigate these concerns, Sorbacal® A has emerged as a superior sorbent, significantly outperforming standard Ca(OH)₂. Its improved performance can be attributed to its fine particle size and large surface area, which contribute to an enhanced pore structure and increased reaction rates [7]. This makes it particularly effective at reducing acid gas emissions. Given the high variability of chloride levels in waste incineration, a reliable DSI system is essential for ensuring compliance with emission standards. According to Dal Pozzo et al. [8], DSI technology offers a more straightforward solution for SO₂ removal, as it eliminates the need for wet slurry and liquid waste treatment systems, as well as the extensive hardware associated with wet scrubbing methods. This leads to lower capital costs and a more compact system design. In addition, the current emphasis on reducing both NO_x and SO₂ emissions has increased interest in DSI processes that can remove both pollutants using a single, cost-effective sorbent. Such a process has recently been developed, involving the injection of Ca(OH)₂ into the upper furnace region of a boiler [9]. As mentioned by Dal Pozzo et al. [8], this method can create a fine, dry powder composed of Ca(OH)₂ and lime, which can simultaneously remove NO_x, HCl, HF, SO₂ and SO₃. The relative removal efficiency of gaseous emissions can be adjusted to meet specific site needs by varying the lime concentration in the hydration process [7].

Beylot et al. [9] mentioned that, for flue gases containing 2500 ppm of SO₂ and 750 ppm of NO_x, the Ca(OH)₂ system operates with a molar ratio of lime to calcium of 0.075. This ratio produces a calcium-to-sulfur (Ca/S) ratio of 2 and a lime-to-NO_x ratio of 0.5, achieving a mass injection rate only 6% higher than Ca(OH)₂ alone. This lime hydrate is particularly effective at removing SO₂ when injected into the upper furnace of a boiler, where gas temperatures range from 850℃ to 1200℃. The lime component reacts at temperatures between 850℃ and 1100℃, primarily producing nitrogen (N₂) and water vapour (H₂O) [10]. After injection, the spent calcium portion of the sorbent can be captured and removed using standard equipment, such as precipitators or baghouses. The high temperature of the furnace also allows for the removal of sulfur trioxide (SO₃) through reactions with calcium [10]. In a study by Beylot et al. [9], dry SO₂ removal methods using Ca(OH)₂ typically achieved a SO₂ reduction of 60% at Ca/S ratios of 2. Attempts to enhance SO₂ removal efficiency and improve calcium utilization through modifications to the hydrate properties resulted in limited improvements [9]. However, rather than focusing solely on increasing SO₂ removal and calcium utilization, the appeal of dry SO₂ removal techniques was further improved by developing a single dry sorbent capable of removing both SO₂ and NO_x. In an effort to make this process more cost-effective, a modified hydrate has been designed to simultaneously capture both SO₂ and NO_x [9].

This study was conducted to evaluate the effectiveness of Ca(OH)₂ injections in MSWI, to reduce acid gas stack emissions below recommended limits of PM10: 30 mg/Nm³ (24-hour average), SO₂: 50 mg/Nm³ (24-hour average), HF: 01 mg/Nm³ (24-hour average), NO₂: 100 mg/Nm³ (1-hour average), CO: 50 mg/Nm³ (1-hour average), HCl: 10 mg/Nm³ (24-hour average), and Total organic carbon (TOC): 10 mg/Nm³. This study aims to address a significant gap in the literature by focusing on the MSWI sector, which has faced challenges in complying with the new regulatory standards, following the amendment of Section 21 of the National Environmental Management: Air Quality Act (NEMA: AQA) No. 39 of 2004. While related studies have been conducted in industries such as steel, as highlighted by Laird et al. [11], there is a notable lack of research exploring the application of Ca(OH)₂ in MSWI. This study seeks to bridge that gap by evaluating the effectiveness of Ca(OH)₂ at temperatures between 850℃ and 1200℃, offering a potential solution for reducing acid gas emissions and assisting the MSWI sector in meeting increasingly stringent environmental standards.

3.1 Sorbent properties evolution with hydration time/ Ca(OH)₂

The evolution of sorbent properties during hydration can be observed in terms of changes in particle size distribution, surface area, and porosity. Table 1 and Figure 2 summarize the results of the particle size study, showing the progression of particle size with hydration time. From Figure 2, the image displays the changes in particle morphology, highlighting the increased surface area and porosity in the post-modification sample (b) with 22 m²/g surface area compared to the pre-modification sample (a) with14 m²/g surface area.

Initially, a bimodal size frequency curve was observed after 3 hours of slurrying, suggesting that a significant portion of the raw materials remain unreacted, leading to a mixture of partially reacted solids. However, at longer hydration times, the particle size distribution became more uniform, with similar curves seen at 9 and 24 hours, where the mass mean diameter (D=4.3) reduced to 12.8 µm and 12.2 µm, respectively, indicating a decrease in particle size as hydration progressed. This reduction in particle size, along with longer slurrying times, contributed to an increase in surface area. While the primary contributor to surface area in porous materials was porosity, the observed reduction in particle size also played a role. Additionally, the BET surface area increases alongside the hydration of Ca(OH)₂, as shown in Figure 2, where nearly complete conversion of Ca(OH)₂ was achieved after prolonged hydration similar to the findings by Zhu et al. [12].

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Figure 2. SEM images of Ca(OH)₂ samples before and after modification

The mesopore volume also showed a positive correlation with surface area, particularly for higher hydration times, where values for BET surface area (79.2 and 64.6 m²/g) correspond to larger mesopore volumes. This indicated that microporosity plays a significant role in enhancing surface area, which can improve the sorbent’s overall performance, particularly for applications involving reactions like desulfurization that occur predominantly in the mesopore region (around 6 nm in diameter). The physical properties of Ca(OH)₂ shown in Table 1, including a BET surface area of 22 m²/g, pore volume of 1.894 cm³/g, macropore volume of 1.791 cm³/g, and mesopore volume of 0.103 cm³/g, along with a skeletal density of 1.895 g/cm³ and a mean volume diameter of 4.852 µm, align with the expected trends seen during sorbent hydration. These characteristics highlight the potential of Ca(OH)₂ as a sorbent, with increasing surface area and porosity supporting improved reactivity and application efficiency. Hydration affects particle size by typically causing a reduction in the overall size of particles. During the hydration process, chemical reactions, such as the reaction between water and materials like Ca(OH)₂, lead to changes in the structure of the particles. As hydration progresses, unreacted raw materials dissolve or react, leading to the formation of smaller, more uniform particles [12]. Zhu et al. [12] stated that in the early stages of hydration, particles may be larger due to incomplete reactions, with a mixture of both reacted and unreacted materials. Over time, as hydration continues and more of the raw materials react, the particle size distribution becomes more uniform, and the mean particle size decreases.

3.2 The impact of Ca(OH)₂ in reducing gaseous emissions

3.2.1 HCl

The implementation of Ca(OH)₂ injections has proven highly effective in reducing HCl emissions from waste incineration processes, as demonstrated by data collected before and after the intervention. Figure 3 illustrates a significant decrease in HCl emissions, from a peak of 417 mg/Nm³ in April 2015 to a low of 8 mg/Nm³ in June 2017. Post-intervention, the recorded levels remained consistently below the recommended threshold of 10 mg/Nm³, with only minor exceedances observed in early 2017. A Wilcoxon signed-rank test produced a p-value of 0.002, indicating a statistically significant reduction in HCl levels following the treatment. Additionally, the standard deviation decreased substantially from 53.907 to 2.044, indicating more consistent control of emissions within the desired limits.

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Figure 3. Comparison of hydrogen chloride findings between 2018/19 and 2020/21

Several factors contributed to the successful reduction of HCl emissions, particularly the properties and application techniques of Ca(OH)₂. The fine particle size (less than 50 µm) and high surface area of the sorbent enhanced its reactivity and adsorption efficiency, both critical characteristics given the short residence times typical of incineration processes. These conditions necessitate rapid and efficient chemical reactions to achieve high conversion rates. Notably, in January 2021, high Ca(OH)₂ consumption rates were required to meet stringent HCl reduction targets, although this led to operational challenges, such as blockages in the lime feeder system. These challenges underscore the importance of optimizing feed rates and maintaining proper system functionality to ensure high efficiency without compromising the system's integrity. Variations in reduction efficiency over the study period were also influenced by combustion conditions, including oxygen levels and temperature. For example, higher oxygen levels in the flue gas during January 2021 facilitated more effective HCl reduction, even though lime consumption was elevated. These findings are consistent with Beylot [9], who highlighted the role of calcination conditions—such as temperature, reaction time, and raw material quality—in improving HCl adsorption efficacy.

Environmental factors, such as relative humidity and the presence of gases like CO₂, also played a role in HCl reduction efficiency. Zhu et al. [12] observed that high humidity and CO₂ concentrations could hinder HCl adsorption, a finding corroborated by this study. Heavy wet infectious waste, for example, contributed to less effective HCl reduction in certain instances. The optimal conditions for HCl adsorption identified in this study—a temperature of 1200℃ and a lime injection rate of 3.2 kg/h—differ from those reported by Sarkar [13], who found a maximum conversion of 71% at 850℃. This discrepancy highlights the importance of adjusting operational settings and material characteristics to suit specific incineration conditions and emissions targets. The data clearly demonstrate that Ca(OH)₂ injections are an effective method for controlling HCl emissions, achieving reductions of up to 97%. However, the study also emphasizes the need for careful management of injection processes and maintenance to sustain optimal performance and avoid operational issues.

During the study, it was observed that upon the release of organic volatiles, the sorbents underwent calcination, converting to calcium oxide (CaO) at approximately 700℃. Once formed, CaO reacted with HCl according to the reaction: CaO + 2HCl ⇌ CaCl₂ + H₂O, a process critical to gas scrubbing and pollutant removal, particularly in waste incineration and flue gas treatment where HCl was a common by-product. At lower temperatures, such as 600℃, a direct reaction between HCl and calcium carbonate (CaCO₃) also occurred, leading to the decomposition of CaCO₃ and the production of calcium chloride (CaCl₂), carbon dioxide (CO₂), and water (H₂O). The calcination of CaCO₃ released CO₂, and the resulting CaO continued to react with HCl, a process similar to that reported by Zhang et al. [14]. Both the high-temperature CaO-HCl reaction and the lower-temperature CaCO₃-HCl reaction were vital for neutralizing acidic gases in incineration processes. The efficiency of these reactions was influenced by several factors, including temperature, sorbent surface area, and gas flow rates, all of which affect the sorbent's capacity to capture acidic gases effectively. These findings highlight the temperature-dependent reactivity of calcium-based sorbents, particularly in flue gas desulfurization and other emission control applications.

3.2.2 HF

The intervention involving Ca(OH)₂ injections led to a substantial reduction in HF emissions, with the data clearly showing compliance with air quality standards post-intervention. Figure 4 illustrates the significant drop in HF concentrations, from a peak of 2 mg/Nm³ prior to the intervention to a maximum of 0.5 mg/Nm³ afterward, well below the recommended limit of 1 mg/Nm³. A Wilcoxon signed-rank test comparing HF emissions from 2014/15 to 2016/17 yielded a p-value of 0.004, confirming the effectiveness of the Ca(OH)₂ treatment. The mean HF concentration decreased from 1.6 mg/Nm³ to 0.1 mg/Nm³, with a notable reduction in standard deviation, indicating more consistent control over emissions following the intervention. The effectiveness of Ca(OH)₂ in reducing HF emissions can be attributed to its fine particle size and large surface area, which increased its reactivity, especially given the short residence times typical of the combustion process. The operational conditions, such as combustion temperature and waste segregation practices, were also critical in achieving high HF reduction efficiencies. Optimal HF adsorption occurred at temperatures between 1100℃ and 1200℃, with a Ca(OH)₂ injection rate of 3.0 kg/h. Proper waste segregation at healthcare facilities, particularly the exclusion of aerosol spray cans from the waste stream, significantly contributed to the reduction of HF emission levels. Studies by Su et al. [15] and Dal Pozzo et al. [7] corroborate these findings, highlighting the role of Ca(OH)₂ in effective dry and semi-dry neutralization processes for acid gases like HF. These studies also emphasize the adaptability of Ca(OH)₂ in various operational conditions and waste compositions.

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Figure 4. Comparison of hydrogen fluoride findings between 2018/19 and 2020/21

During the study, it was observed that, at injection temperatures above 600-800℃, Ca(OH)₂ decomposed into calcium oxide (CaO), which then reacted with HF in areas where the temperature had dropped below 600-800℃. This reaction, proceeding through the intermediate formation of CaO, aligns with the findings by Dowling et al. [16]. The decomposition of Ca(OH)₂ into CaO at higher temperatures enhanced its reactivity, as CaO has a higher affinity for HF. This allowed for more efficient gas capture in processes like flue gas desulfurization or gas scrubbing. The efficiency of the reaction depended on factors such as the residence time of reactants, cooling rates, and the temperature gradient in the system, all of which influenced CaO’s reactivity with HF and other acidic gases. Additionally, the increased surface area of CaO facilitated more effective gas-solid reactions, improving sorbent performance. Despite the significant reduction in HF emissions, operational challenges were noted, such as potential system blockages due to lime build-up. This underscores the importance of regular maintenance and system checks to maintain efficiency. The reaction dynamics of Ca(OH)₂ with HF, particularly the temperature dependency and stoichiometric ratios, emphasize the need for precise control over injection rates and environmental conditions within the incinerator. The data from this study demonstrate that Ca(OH)₂ injections are highly effective in reducing HF emissions from healthcare waste incineration, achieving an 86% reduction in HF levels. This intervention not only met but exceeded environmental standards, offering a robust framework for similar applications in waste management and emission control, particularly in environments requiring strict regulatory compliance.

3.2.3 SO₂

The data collected before and after the Ca(OH)₂ intervention demonstrates a substantial reduction in SO₂ emissions. Figure 5 depicts these reductions: prior to the intervention, SO₂ concentrations reached a peak of 378 mg/Nm³ in May 2018, but following the intervention, the highest recorded level dropped to 70 mg/Nm³ in January 2021. By April 2021, SO₂ levels had decreased to as low as 20 mg/Nm³, consistently remaining below the recommended limit of 50 mg/Nm³ for much of the study period. The Wilcoxon signed-rank test results presented a p-value of 0.003, strongly indicates the statistical significance of the reduction, highlighting the efficacy of Ca(OH)₂ treatment. The reduction in mean SO₂ concentration, from 316.7 mg/Nm³ before the intervention to 39.9 mg/Nm³ post-treatment, not only demonstrates the effectiveness of Ca(OH)₂ but also highlights its potential as a versatile solution in emission control strategies. This is consistent with the findings reported by Flores et al. [17]. The successful reduction in SO₂ emissions can be attributed to several key factors, primarily the fine particle size and high surface area of Ca(OH)₂, which enhanced its adsorption capacity [16]. The efficiency of SO₂ capture exceeding 87%, was optimized by controlling reaction conditions such as temperature and moisture content in the flue gas. The optimal adsorption temperature for SO₂ was found to be around 1020℃, which aligns with the enhanced reactivity of the sorbent at elevated temperatures. Moreover, the competition between SO₂ and other acidic gases, such as HCl, for adsorption sites on Ca(OH)₂ particles necessitated precise management of injection rates and environmental conditions to maximize the efficiency of each targeted emission reduction.

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Figure 5. Comparison of sulphur dioxide findings between 2018/19 and 2020/21

A key observation was that when the initial SO₂ concentration in the injection stage was reduced from 3,000 ppm to 1,500 ppm, the sorbent's conversion rates declined. For instance, at 510℃/454℃, the ratio of initial sulfation conversion for the reduced SO₂ concentration compared to the reference condition was only 0.84, while at 538℃/482℃ and 566℃/510℃, the ratios were 0.74 and 0.78, respectively. The lower SO₂ concentration allowed more of the Ca(OH)₂ to react with CO₂, increasing carbonation in the initial injection stage, this aligns with the findings reported by Sarkar [13]. As the sorbent progressed to the filtration stage, further reactions were dependent on the exposure of active CaO formed by the decomposition of residual Ca(OH)₂. This highlights the dynamic interplay between sulfation and carbonation, where reduced SO₂ levels shifted the balance toward greater carbonation. The presence of CO₂ and other gases also played a significant role in influencing the SO₂ adsorption efficiency. The reactivity of Ca(OH)₂ with both SO₂ and CO₂, coupled with factors like gas flow velocity and humidity, further dictated the performance of the process [15]. When SO₂ concentrations decreased, more sorbent was available to react with CO₂, promoting the formation of calcium carbonate (CaCO₃) in addition to calcium sulfite (CaSO₃) and calcium sulfate (CaSO₄) during the sulfation process.

The findings from this study align with those from Su et al. [15], which highlight the importance of temperature and the chemical makeup of the sorbent in optimizing SO₂ reduction. These studies suggest that selective non-catalytic reduction (SNCR) and wet-spray systems may offer additional benefits for SO₂ control under certain operational conditions. However, the high dosage of Ca(OH)₂ required for competitive absorption with HCl raised concerns about potential increases in operational costs and maintenance needs. Adjustments in particle size and stoichiometric ratios were necessary to navigate the competitive adsorption processes between SO₂, HCl, and CO₂ in the flue gas environment. Operational challenges, such as potential calcium hydroxide build-up leading to system blockages, emphasize the importance of regular maintenance to ensure continuous efficiency [7]. Furthermore, the sorbent’s reactivity in the injection stage and filtration stage was influenced by factors such as residence time and temperature gradients, which affected the exposure and decomposition of Ca(OH)₂ into CaO. Despite these challenges, the introduction of Ca(OH)₂ injections resulted in a dramatic reduction of SO₂ emissions, achieving up to an 87% decrease, thus significantly enhancing air quality compliance. The study suggests that Ca(OH)₂ is a highly effective sorbent for large-scale industrial applications, provided that sorbent and process parameters are optimized to accommodate specific incineration conditions and flue gas compositions. Further research into combined treatment methodologies, such as integrating SNCR with Ca(OH)₂-based systems, could lead to even greater improvements in SO₂ emission control.

3.2.4 NO_x

The reduction of NOₓ emissions through the use of Ca(OH)₂ injections demonstrated significant improvements in emission control. Before the intervention, NOₓ levels reached highs of 289 mg/Nm³, with lows of 180 mg/Nm³ during the 2018/19 period. After the intervention, these levels dropped considerably, with NOₓ concentrations ranging from 50 mg/Nm³ to 30 mg/Nm³, consistently staying below the recommended limit of 200 mg/Nm³, as shown in Figure 6. Statistical analysis using the Wilcoxon signed-rank test, with a p-value of 0.005, confirmed an 83% reduction in NOₓ levels. The average NOₓ concentration after treatment was 41 mg/Nm³ compared to a pre-treatment average of 248.5 mg/Nm³, demonstrating the effectiveness of Ca(OH)₂ injections in controlling NOₓ emissions. Interestingly, the removal of NOₓ had minimal impact on SO₂ removal at lower NOₓ concentrations. However, as NOₓ removal rates increased, a negative effect on SO₂ removal was observed, similar to the findings by Dal Pozzo et al. [8]. Experiments using a reactive mixture of Ca(OH)₂ and CaSO₃·0.5H₂O combined with 10% sodium hydroxide (NaOH) showed better SO₂ removal when NOₓ was absent. At temperatures of 860℃ and 920℃, NOₓ removal remained relatively high, with rates of 1.3 and 3.2, respectively. When SO₂ concentrations were increased, NOₓ removal improved, but in the absence of SO₂, NOₓ removal was even higher, likely due to competition between SO₂ and NOₓ for reactive pore sites on the sorbent. According to Dal Pozzo et al. [8], SO₂ may inhibit NOₓ removal by occupying more active sites, while the presence of CaSO₃ aids in NOₓ removal.

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Figure 6. Comparison of oxide of nitrogen findings between 2018/19 and 2020/21

For the Ca(OH)₂ and CaSO₃ mixture at 0.08 M NaOH, NOₓ removal without SO₂ was greater (1.6) compared to conditions with 200 ppm SO₂ (1.1). As SO₂ concentrations increased to 500 ppm and 1500 ppm, NOₓ removal improved to 3.2 and 4.1, respectively. These results emphasize the need to carefully balance the concentrations of SO₂ and NOₓ in industrial processes to maximize the removal efficiency of both pollutants. NO_x removal, measured in moles of NO_x per mole of Ca(OH)₂, was relatively constant across varying concentrations, except when using highly reactive solids at 200 ppm NO_x. The addition of Ca(OH)₂ at temperatures around 850℃ played a critical role in capturing NOₓ, which is typically produced through reactions involving heat, fuel, and available oxygen. NOₓ reduction was highly temperature-dependent, with optimal results achieved when Ca(OH)₂ was injected at higher temperatures [11]. This highlights the importance of controlling operational temperatures to maximize NOₓ reduction. The study also noted interactions between NOₓ reductions and the concentrations of other gases, particularly SO₂ and O₂. Increased O₂ levels enhanced the removal of both NOₓ and SO₂, indicating a synergistic effect that facilitates more efficient scrubbing of emissions. Changes in the Ca(OH)₂ injection rate improved NOₓ reductions and simultaneously enhanced SO₂ capture, optimizing reagent use and reducing operational costs.

Supporting study by Dal Pozzo [8] demonstrated similar improvements in NOₓ and SO₂ reductions under controlled experimental conditions, highlighting the potential of optimized sorbent injection strategies in achieving significant emission reductions. However, the temperature-sensitive nature of the reactions posed operational challenges. Balancing the temperature conditions to effectively manage both NOₓ emissions proved complex, especially in the presence of CO₂, O₂, and other components in the flue gas. These factors influenced the adsorption efficiency of Ca(OH)₂, emphasizing the need for precise control over gas composition during emission control processes. The use of Ca(OH)₂ injections was highly effective in significantly reducing NOₓ emissions, achieving an 83% reduction. Additionally, interactions between SO₂ and NOₓ removal highlight the importance of optimizing operational parameters, such as temperature, injection rates, and gas compositions, to manage both emissions efficiently. While Ca(OH)₂ proved to be a versatile sorbent, further research into strategies for simultaneously controlling NOₓ and SO₂ could provide insights into even more effective methods for emission reduction in industrial applications.

3.3 Factors affecting calcium hydroxide efficiency

3.3.1 The effect of resident time

The effect of residence time plays a pivotal role in the efficiency of gas-phase reactions for the removal of acid gases such as SO₂ and NO_x when using Ca(OH)₂ [18]. This interaction between the sorbent and the pollutants is governed by both the chemical kinetics of the reactions and the physical transport processes. Increasing residence time enhances the opportunity for these interactions, leading to more complete reactions and thus improving removal efficiencies [19]. It was observed that for SO₂ removal, the primary reaction involves the formation of CaSO₃ and CaSO₄ through the reaction between SO₂ and Ca(OH)₂ as presented in Eq. (5). Prolonging the residence time allowed for greater diffusion of SO₂ onto the surface of the Ca(OH)₂ particles, enabling this reaction to proceed further to form calcium sulfate:

$\mathrm{CaSO}_3+1 / 2 \mathrm{O}_2 \rightarrow \mathrm{CaSO}_4$         (8)

The process of SO₂ removal typically required more residence time because it involved multiple stages. A longer residence time 0.8 s ensured that SO₂ was fully captured by the sorbent, as the conversion to CaSO₄ took additional time compared to other gas-phase reactions.

In contrast, NO_x removal occurred through reduction reactions, where NO and NO₂ were reduced to nitrogen (N₂) in the presence of reducing agents Ca(OH)₂, with the help of ammonia. The reaction for NO removal was:

$4 \mathrm{NH}_3+4 \mathrm{NO}+\mathrm{O}_2 \rightarrow 4 \mathrm{~N}_2+6 \mathrm{H}_2 \mathrm{O}$     (9)

This reaction tended to be faster than SO₂ removal because it followed a simpler reaction mechanism. As a result, NO_x removal achieved significant reductions in a much shorter residence time, as evidenced by the completion of NO_x removal in approximately 200 ms, with a 50% removal efficiency. On the other hand, SO₂ removal, required more complex interactions, reached completion after about 400 ms with a 47% removal efficiency (Figure 7). This is similar to the reaction reported by Jones [20]. The residence time is crucial in these systems, where a total residence time of 1 second allowed for all the reactions to proceed to completion. This ensured that all chemical interactions between the sorbent and pollutants, particularly those involving calcium hydroxide, are fully realized [14]. Prolonged residence time also facilitated secondary reactions, such as the oxidation of CaSO₃ to CaSO₄, which improves the efficiency of SO₂ removal further. Without sufficient residence time, pollutants could exit the reactor before full neutralization occurs, decreasing the overall removal efficiency [18]. While CaO is also reactive, it has a slightly lower reactivity than Ca(OH)₂, which can affect the overall efficiency of SO₂ removal. Despite this, the extended residence time of 1 second ensured that the necessary reactions still proceed effectively. In conclusion, residence time is a fundamental parameter in optimizing the removal of gaseous emissions using Ca(OH)₂. A longer residence time allowed the sorbent to fully react with the pollutants, ensuring that both SO₂ and NO_x are efficiently captured. The reactions involved, such as the formation of calcium sulfite and sulfate for SO₂ and the reduction of NO_x to nitrogen, highlight the importance of allowing sufficient time for these processes to occur.

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Figure 7. The differing reaction rates and efficiencies of NO_x and SO₂ removal as a function of residence time

3.3.2 Effect of temperature

In this study, the effect of temperature and the calcium hydroxide injection rate, represented by the Ca/S and N/NO_x ratios, were identified as key factors significantly influencing the removal efficiency of SO₂ and NO_x. The removal efficiency varied with temperature, with SO₂ removal reaching its peak efficiency at approximately 1100℃ and NO_x removal peaking at around 950℃. At these optimal temperatures, the maximum NO_x removal reached 83%, while SO₂ removal achieved 87%, observed with Ca/S and N/NO_x ratios of 3 and 1.5, respectively. The study also demonstrated that higher injection rates improve both SO₂ and NO_x removal efficiencies. For instance, increasing the Ca/S ratio from 2 to 3 resulted in a significant improvement in SO₂ reduction, while increasing the N/NO_x ratio from 1 to 1.5 similarly boosted NO_x removal. However, as the temperature dropped below 950℃, ammonia (NH₃) emissions began to increase sharply, posing a challenge for emissions control. These findings are consistent with the findings reported by Dowling et al. [16]. It was further observed that above 950℃, NH₃ emissions remained low (below 10 ppm), regardless of the N/NO_x ratio, making this temperature range ideal for optimizing the trade-off between NO_x and NH₃ emissions. Based on these findings, different operational strategies were adopted, at higher temperatures (1100℃) optimized SO₂ removal, while a temperature near 950℃ optimized NO_x reduction. A compromise approach was reached at around 1000℃, balancing the reduction of both pollutants, although it was not able to optimize the removal of either pollutant fully.

The temperature reaction curve (Figure 8) displayed below further illustrates the relationships between SO₂, NO_x removal, and NH₃ emissions across the tested temperature range. It was observed that the injection temperature of 950℃ played an important role in accelerating the reaction kinetics, particularly for NO_x, as higher temperatures aid in the dissociation of nitrogen oxides. At such elevated temperatures, Ca(OH)₂ begins to decompose into CaO and water, a reaction represented in Eq. (1). This decomposition influenced the overall efficiency of SO₂ removal, although extended residence times and optimal temperatures ensured that necessary reactions still proceeded effectively for effective gas treatment [20]. The decomposition of Ca(OH)₂ into CaO affected the availability of active sites for the removal processes and changed the chemical dynamics within the reactor. However, the formation of CaO still contribute to SO₂ removal through secondary reactions with the gas. This highlights the importance of controlling the temperature within specific ranges to maintain the integrity of Ca(OH)₂ and optimize its use for pollutant removal, especially in high-temperature environments where rapid kinetics are crucial.

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Figure 8. The effect of temperature and hydrate injection rate on SO₂ and NO_x removal using Ca(OH)₂

3.3.3 Effect of CO₂ concentration

Although CaCO₃, formed from the reaction between Ca(OH)₂ and CO₂, can further react with SO₂ in the medium temperature range, high levels of sorbent carbonation were identified as a major limitation in achieving optimal sorbent utilization. This is primarily due to the lower sulfation rate of CaCO₃ compared to CaO [20]. To investigate whether reducing CO₂ concentrations could enhance sorbent sulfation conversion, the CO₂ concentration during Ca(OH)₂ injection was reduced from 12% to 6% (Figure 9). When comparing sorbent conversion curves for the reduced CO₂ concentration to those under the reference gas conditions, no significant differences were observed in the starting points of the curves. This finding suggested that reducing CO₂ concentration had no noticeable effect on the sulfation conversion of Ca(OH)₂ during the injection stage. This interpretation, however, does not align with the observations reported [7, 8, 18]; which noted a significant conversion curve when the CO₂ concentration was reduced from 14% to 7%. As a result, SO₂ concentrations measured in the filtration chamber were similar for both the reduced CO₂ condition and the reference gas condition. This is consistent with the findings reported by Dal et al. [18]. This observation indicated that the initial sulfation conversion of the sorbent in the injection stage is most likely diffusion-controlled. The rate at which SO₂ diffuses to the reactive surface of the sorbent determined the degree of sulfation conversion. Since the diffusion flux of SO₂ remained unaffected by the reduction in CO₂ concentration, the sulfation rate of Ca(OH)₂ in the injection stage was unchanged. Previous research by Flores et al. [17] on the effects of particle size on Ca(OH)₂ reactions also supports this conclusion, noting that bulk diffusion of SO₂ can be a controlling factor unless the sorbent particles are smaller than 5 µm in mass median diameter.

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Figure 9. The effect of CO₂ concentration on the reaction of Ca(OH)₂ during the injection and filtration stages

Despite halving the CO₂ concentration, the gas remained much more abundant than SO₂. Consequently, while the carbonation reaction in the injection stage was governed by the reaction rate, the reduction in CO₂ concentration led to a decrease in initial carbonation conversions. However, since the decomposition of Ca(OH)₂ into CaO was unaffected by the lower CO₂ concentration, and less CO₂ reacted with the resulting CaO, higher dehydration conversions were achieved in the injection stage compared to the reference gas condition [16]. In the filtration stage, both SO₂ and CO₂ competed for the gradually exposed reactive surface of the sorbent. Due to the faster reaction rate of SO₂ compared to CO₂, reducing the CO₂ concentration only slightly affected sorbent conversions during the early stages of filtration. Similar to Neuwahl et al. [19], the initial sulfation rates were marginally higher, while carbonation rates were slightly lower when the CO₂ concentration was halved. However, the final sulfation conversion values for the sorbent were almost identical to those observed under the reference gas conditions. The overall sorbent conversion curves for the reduced CO₂ concentration were largely similar or parallel to those of the reference case. Based on the study, it is evident that minor alterations in CO₂ concentration do not significantly affect the SO₂ or other gases removal efficiency of the process. It can be concluded that, while CO₂ concentration influenced the initial carbonation reaction, the overall effectiveness of Ca(OH)₂ in capturing gases remained relatively stable, with the process primarily governed by the diffusion-controlled sulfation reaction.

3.4 Practical considerations and challenges of Ca(OH)₂ injection in MSWI

While the results demonstrate the effectiveness of Ca(OH)₂ in reducing acid gas emissions, several challenges were observed when implementing this solution in real-world waste incineration facility. One of the primary concerns was the operational management of Ca(OH)₂ injection systems. The fine particle size of Ca(OH)₂ enhanced its reactivity but also increased the risk of system blockages, particularly in the lime feeder and injection systems. Maintaining a consistent feed rate was crucial to avoid overconsumption of the sorbent, which led to increased operational costs and equipment malfunctions. Regular maintenance and system calibration were essential to ensure the smooth functioning of the injection system and to prevent downtime caused by lime build-up or feeder blockages. Another significant challenge was optimizing the injection rates and sorbent distribution throughout the flue gas stream to ensure maximum contact between the sorbent and the acid gases. Inadequate mixing led to uneven distribution, reducing the overall efficiency of the gas scrubbing process. This required careful design and engineering of the injection system, considering factors such as gas flow velocity, temperature gradients, and reaction kinetics. Additionally, variations in waste composition, such as higher moisture content in waste impacted the efficiency of Ca(OH)₂ in reducing HCl and SO₂ emissions, necessitating adjustments in operating parameters.

3.5 Economic feasibility of Ca(OH)₂ injections

The cost of the sorbent itself is relatively low compared to other emission control technologies such as wet scrubbers or selective catalytic reduction systems. However, the overall cost-effectiveness of this solution depends on several factors, including sorbent consumption rates, the cost of storage and handling, and the expenses related to system maintenance. Ca(OH)₂ injections are attractive due to their lower capital expenditure requirements compared to more complex gas cleaning systems [18]. The compact nature of the injection equipment makes it easier to retrofit into existing waste incineration plants and reduce upfront installation costs. However, high sorbent consumption can drive up operating expenses, particularly if the process is not optimized. Facilities may need to invest in advanced control systems to ensure precise injection rates and to minimize waste, which could increase the initial investment. Additionally, the financial benefits of using Ca(OH)₂ must be weighed against the cost savings from compliance with environmental regulations. By reducing acid gas emissions, facilities can avoid costly fines and meet the increasingly stringent air quality standards set by regulatory bodies. There is also potential for long-term savings from reduced wear and tear on equipment, as acid gas scrubbing reduces corrosion and the need for frequent repairs.