Chien-Chang Liao1, Jhong-Lin Wu2, Ya-Fen Wang3, Kuo-Lin Huang  4, Shih-Wei Huang5,6, Yi-Ming Kuo This email address is being protected from spambots. You need JavaScript enabled to view it.7 

1 Department of Hepatogastroenterology, Tainan Municipal Hospital (Managed by Show Chwan Medical Care Corporation), Tainan 701, Taiwan
Environmental Resource Management Research Center, Cheng Kung University, Tainan 709, Taiwan
Department of Environmental Engineering, Chung Yuan Christian University, Taoyuan 32023, Taiwan
Department of Environmental Engineering and Science, National Pingtung University of Science and Technology, Pingtung 912, Taiwan
Institute of Environmental Toxin and Emerging Contaminant, Cheng Shiu University, Kaohsiung 833, Taiwan
Center for environmental Toxin and Emerging Contaminant Research, Cheng Shiu University, Kaohsiung 833, Taiwan
Department of Safety Health and Environmental Engineering, Chung Hwa University of Medical Technology, Tainan 717, Taiwan


Received: September 16, 2023
Revised: January 15, 2024
Accepted: January 19, 2024

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.


Download Citation: ||https://doi.org/10.4209/aaqr.230192  

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Cite this article:

Liao, C.C., Wu, J.L., Wang, Y.F., Huang, K.L., Huang, S.W., Kuo, Y.M. (2024). Advanced Scrubbing/Filtration Devices for Pollution Control in Medical Laboratory Waste Incineration System. Aerosol Air Qual. Res. 24, 230192. https://doi.org/10.4209/aaqr.230192


HIGHLIGHTS

  • The advanced scrubbing/filtration system provided excellent control of condensable particulate matter.
  • The corrosion caused by HCl was eliminated thanks to the two-stage scrubbing process.
  • Integration of waste liquid injection with the incinerator improves the efficiency of combustion.
 

ABSTRACT


This research investigated the release of pollutants from a medical laboratory waste incineration system and assessed the effectiveness of advanced scrubbing and filtration devices. Samples of flue gas, fly ash, bottom ash, scrubbing wastewater, and sludge were collected for analysis. By introducing liquid waste solvent into the primary combustion chamber, it is possible to achieve an average incineration temperature of over 950°C, ensuring a combustion efficiency of 99.93%. The study findings indicate that the use of advanced scrubbing/filtration devices effectively removed pollutants, resulting in pollutant concentrations in the stack flue gas that met emission standards. The system effectively protects the fabric filter against acid gas corrosion, as demonstrated by the removal of HCl at a rate exceeding 99.99%. Additionally, the two-stage scrubbing process involves converting condensable particulate matter (CPM) into filterable particulate matter (FPM) by reducing the temperature of the flue gas to 70°C. The fabric filter effectively removed FPM in a sequential manner. Therefore, the advanced scrubbing/filtration devices demonstrated exceptional efficacy in removing FPM and CPM, with removal efficiencies surpassing 99.8%.


Keywords: Filterable particulate matter, Condensable particulate matters, Incineration system, Medical laboratory waste


1 INTRODUCTION


A wide range of laboratory waste (LW) is generated in educational institutions as a result of research and educational endeavors. Disposing of this waste presents a challenge due to its diverse nature and small quantities. In order to tackle this concern, the Sustainable Environment Research Center at National Cheng Kung University established a LW treatment plant in 2005. The facility comprises three primary treatment systems, namely an incineration system, a physicochemical treatment system, and a plasma melting system. The incineration system has been specifically engineered to effectively combust medical laboratory waste (MLW) and plastic containers. It has been reported in a study conducted by Coutinho et al. (2006) that MLW frequently exhibits elevated concentrations of chlorine. A previous study conducted by Wang and Chang-Chien (2007) revealed that the incineration process can produce a substantial quantity of air toxics, specifically polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), particularly when the input materials contain a high concentration of chlorine.

Particulate matter, in addition to air toxics, is a prevalent pollutant that significantly contributes to air pollution and has garnered significant public concern. Particulate matter is comprised of two types: filterable particulate matter (FPM) and condensable particulate matter (CPM). FPM can be efficiently managed through the utilization of existing air pollution control technologies. However, it should be noted that CPM remains in the vapor phase within the flue gas and undergoes a conversion process into particulate matter or aerosols upon its release into the atmosphere (Wu et al., 2020). CPM is a matter of significant concern due to its ability to bypass conventional air pollution control devices (APCDs) without being effectively captured (Feng et al., 2018). Once the conversion into the solid phase occurs, the particulates or aerosols have the potential to make a substantial contribution to air pollution and diminish atmospheric visibility. Therefore, it is imperative to find a solution for the elimination of CPM. In recent years, various techniques have been utilized to eliminate CPM, including low-temperature electrostatic precipitators, electrostatic-bag-precipitators, wet electrostatic precipitators, and wet flue gas desulfurization (Li et al., 2019; Wang et al., 2020; Wang et al., 2022).

While the use of APCDs in a treatment plant can effectively remove conventional air toxics from the flue gas, it is important to note that the resulting fly ash and bottom ash still contain significant amounts of PCDD/Fs. Therefore, additional treatment is necessary, such as employing a plasma melting system, to address these high levels of PCDD/Fs. The plasma melting system has been specifically engineered to provide stabilization for various hazardous residues, such as sludge, bottom ash, fly ash, and other types of waste, originating from incineration and physicochemical treatment systems (Kuo et al., 2010). The process of plasma melting involves the encapsulation of hazardous residues within an amorphous and glassy matrix, thereby converting them into materials that can be recycled (Kuo et al., 2011).

Due to the corrosive effects of hydrochloric acid (HCl) on the APCD process in the treatment plant underwent renovation five years ago. A previous study conducted by Kuo et al. (2020) has shown that the renovated system resulted in an enhanced removal of PCDD/Fs. However, the primary objective of this study was to examine the influence of APCD units on the elimination of fine particulates and other pollutants. We conducted measurements of CPM, FPM, total non-methane hydrocarbons (TNMHCs), and acid gases at various sampling locations within the APCDs in order to assess their respective removal efficiencies. We also examined the relative proportions of CPM and FPM in the atmospheric aerosols.

 
METHODS


 
2.1 Processes of Incineration System

Fig. 1 presents the process flow diagram of incineration. The incineration system exhibited a treatment capacity of 375 kg hr1 for the MLW. The MLW primarily originated from the experimental practices implemented in educational institutions, and its structure was characterized by a high degree of complexity. According to the findings of our previous study, the lower heating value of the substance varied between 3,000 kcal kg1 and 4,500 kcal kg1, while the Cl content frequently reached levels as high as 10%. The MLW was introduced into the primary incineration chamber through a conveyor system. To achieve the thorough degradation of hazardous organic compounds, the primary incineration chamber was maintained at a temperature of 900°C while operating with a residual oxygen level of 12%. Furthermore, an organic liquid LW with a lower heating value higher than 5,000 kcal kg1, was introduced into the chamber from the upper section. This injection aimed to establish a localized high-temperature setting (> 1400°C) at the lower portion of the chamber, thereby facilitating the fusion of the bottom ash. The molten material underwent a quenching process and subsequently transformed into inert slag, which was then stored in a designated slag pit.

  Fig. 1. Process diagram of the incineration system.Fig. 1. Process diagram of the incineration system.
 

To facilitate the decomposition of unburned organic gases and CO, the flue gas was subjected to additional heating, raising its temperature to above 900°C. This was achieved by utilizing a diesel burner with an excess air coefficient of 1.17, which ensured the maintenance of an oxygen-rich atmosphere within the secondary incineration chamber. The quantity of flue gas produced varied between 6,000 and 7,500 Nm3 hr1, while the retention time of the flue gas in the primary and secondary incineration chambers was consistently maintained at a minimum of 2.5 s and 1.5 s, respectively.

The research conducted by Kuo et al. (2020) revealed that the simultaneous application of an extended flue gas retention period and increased temperature successfully enhanced the degradation of particulate matter and PCDD/Fs. The flue gas was subsequently directed into the primary quenching tower, where it experienced a gas retention time of 3 s. Throughout the course of this procedure, the gas temperature underwent a decrease from 900°C to 450°C through the utilization of a 40% NaOH solution at a flow rate ranging from 1.5 to 2 tons hr1. The flu gas was then subjected to a two-stage scrubbing process aimed at removing both particulate and soluble pollutants. The liquid-to-gas ratio of the Venturi scrubber was set at 1 L m3, while the injection rate of scrubbing water was 9 m3 hr1. The liquid-to-gas ratio of the packed tower was set at 1.5 L m3, while the injection rate of scrubbing water was established at 14 m3 hr1. The wastewater produced during the two-stage scrubbing process was temporarily stored in a tank situated at the base of the two units. The wastewater in the tank was carefully regulated to maintain a minimum quantity of 25 tons and a pH level greater than 12. This was done to guarantee the effectiveness of the scrubbing process. To mitigate the accumulation of Cl1 and other impurities in the tank, the wastewater produced during the scrubbing procedure was consistently discharged at a rate of 2 tons hr1 to a specialized wastewater treatment system. The wastewater treatment process consists of several essential components, including a cooling unit that is equipped with a heat exchanger, a coagulation/flocculation unit, and a sedimentation tank. The sludge generated in the precipitation unit was conveyed to the plasma melting system for subsequent processing, while the treated water was reused as scrubbing water to minimize water usage.

To mitigate the issue of fabric filter clogging, a dewatering unit was employed to remove moisture in the flue gas. Powdery activated carbon was subsequently introduced into the pipeline at a rate of 1.5 kg hr1 following the dewatering unit, in order to facilitate the adsorption of PCDD/Fs. The flue gas was subsequently passed through a fabric filter in order to remove particulate matter prior to its release into the atmosphere via the stack. The fabric filter consists of 300 filter bags, each with a diameter of 0.16 m and a length of 2 m. The filtering velocity of the fabric filter was intentionally set at 0.8 cm s1.

 
2.2 Collecting and Analyzing Pollutants in Flue Gas

The examination of APCDs included the collection of samples from three distinct locations: the primary quenching tower (A), dewater unit (B), and fabric filter (C). The analysis focused on the concentrations of different pollutants, including as gas pollutants, particulates, TNMHCs, volatile organic compound (VOC) species, and PCDD/Fs. The sampling and analysis procedures adhered to the standard methods prescribed by the Taiwan Ministry of Environment (TME).

The portable gas analyzer (HORIBA PG-250A) was utilized to conduct simultaneous monitoring of NOx, SO2, CO2, CO, and O2. SO2, CO, and CO2 were measured using a non-dispersive gas filter correlation infrared absorption technique, with detection ranges of 0–100 ppmv, 0–1000 ppmv, and 0–5%, respectively (TME, 2006b, 2006c, 2016). NOx was quantified utilizing a chemiluminescence detection method, which had a detection range of 0–250 ppmv (TME, 2006a). O2 concentration was determined utilizing the Galvanic cell analysis method, which had a detection range of 0–21% (TME, 2006d). The flue gases containing HCl, H2SO4, and HNO3 were absorbed using DI water at a flowrate of 2 L min1 for a duration of 10 mins. The specimen was maintained at a temperature below 4°C, introduced into an ionic chromatographer (IC, Dionex, Model DX-120) via a 0.45 µm pore size membrane, and measured using a conductivity detector. The standard solution was introduced concurrently with the sample, resulting in satisfactory recovery within the range of 85–115%.

The sampling and analysis procedure for TNMHCs adhered to the standard method outlined in NIEA A758.70B (TME, 2019a). The gas sample was continuously pumped out from the pipeline using a sampling device and introduced directly into the flame ionization detector for the purpose of measuring the total hydrocarbon carbon. Conversely, a separate portion of the specimen was introduced into a specialized combustion system designed to measure the concentration of TNMHCs, while the methane concentration was determined using flame ionization detector (FID). The TNMHCs was determined by subtracting the CH4 concentration from the TNMHC concentration. The resulting TNMHC concentration was then expressed in terms of methane equivalent (as CH4).

Passive flow controller canisters were employed to collect ambient samples of VOC samples at a consistent flow rate of 40 mL min1, utilizing canisters made of stainless steel coated with silica. The samples were collected during a two-hour time frame, yielding a total volume of 4.8 L. Prior to the sampling process, all canisters underwent cleaning, moisturization, and leak checks to ensure the integrity of the vacuum. Additionally, a laboratory blank analysis was performed to validate the cleanliness of the canister.

The study employed the United States Environmental Protection Agency (U.S. EPA) Method TO-15, which incorporates the Photochemical Assessment Monitoring System and Urban Air Toxics standards, to investigate the composition and concentrations of ambient alkanes, alkenes, aromatics, and halogenated VOCs (101 VOC species). Furthermore, the study adhered to the sampling, preservation, transportation, and analysis protocols outlined by the U.S. EPA Method TO-15 (U.S. EPA, 2014).

Samples were collected utilizing a 16-inlet position autosampler and subjected to pre-concentration using an Entech 7100A prior to being analyzed for polar and non-polar VOCs through gas chromatography (GC; HP6890N, Agilent Technologies, USA) and a mass spectrometry (MS; HP5973N, Agilent Technologies, USA) equipped with DB-VRX capillary columns (length = 60 m, inner diameter = 0.32 mm, and a film thickness = 1.05 µm, Agilent Technologies, USA). The temperature of the GC oven was programmed to initiate at 35°C for a duration of 5 mins, followed by a gradual increase to 230°C at a rate of 5°C min1. Subsequently, the temperature was maintained at 230°C for 3 mins.

The sampling procedure employed for FPM and CPM adhered to the established protocol outlined in NIEA A214.71C (TME, 2019b). The FPM and CPM samples were collected concurrently utilizing a sharp cut cyclone to eliminate particles larger than 2.5 µm. The FPM was collected on a quartz fiber and measured using a 5-digit microbalance (METTLER TOLEDO XS105DU) in a Class-100 clean room. The accumulated FPM on the inner surface was rinsed with acetone and subsequently measured by weight. The increments in mass of the filter and eluent were subsequently divided by the sampling volume (Vs) in order to ascertain the concentration of FPM in the stack. A CPM collection train was arranged behind the FPM sampling unit, comprising of a cooler, impingers, a quartz fiber filter maintained at a temperature of 30°C, a moisture capture impinger, and a silica gel packing column immersed in an iced water bath. The inorganic and organic CPM were collected in the first and second impingers, respectively. The CPM filter sample was subjected to stabilization in a desiccator at a temperature of 30°C for a duration of 24 hours. The sample was weighed both before and after the sampling process. Finally, the cumulative mass increments collected from the impingers and filter were divided by Vs to calculate the CPM concentration. A purging process using pure N2 gas with a purity level exceeding 99.999% was implemented on the sampling train for 1 hour following each sample collection. This procedure was carried out in order to mitigate any potential interference caused by SO3.

 
2.3 Sampling and Analysis of PCDD/Fs in Flue Gas

The stack flue gas emitted by the incineration system was sampled isokinetically over 4 hours. This sampling procedure followed the modified Method 23 of the U.S. EPA as specified in NIEA A807.75C (TME, 2010). The sampling train utilized in this study bears resemblance to the apparatus outlined in the Modified Method 5 of the U.S. EPA. Prior to initiating the sampling procedure, the XAD-2 resin was subjected to enrichment with PCDD/F surrogate standards that had been prelabeled with isotopes. The compounds analyzed in this study included 37Cl4-2,3,7,8-TCDD, 13C12-1,2,3,4,7,8-HxCDD, 13C12-2,3,4,7,8-PeCDF, 13C12-1,2,3,4,7,8-HxCDF, and 13C12-1,2,3,4,7,8,9-HpCDF. The recoveries of surrogate standards for PCDD/Fs were found to be within the acceptable range of 103%–120%, meeting the criteria of 70%–130%. This observation indicates that there was no occurrence of PCDD/Fs during the experiment. These details demonstrate resemblances to the data presented in a previous publication authored by Wang and Chang-Chien (2007).

The analysis procedure for PCDD/Fs in stack flue gases was modified based on U.S. EPA Method 23 and outlined in the NIEA A808.75B Method (TME, 2013). Detailed analytical procedures are outlined in a previous study conducted by Wang and Chang-Chien (2007). Following the extraction process, the initial cleanup was carried out by subjecting the sample to treatment with concentrated sulfuric acid. Subsequently, a column filled with acid silica gel was utilized for the subsequent purification procedure. The provided sample extract was dissolved in 5 mL of hexane and subsequently introduced into the column, accompanied by two additional 5-mL rinses. The column underwent an additional 50 mL of hexane elution, and the complete eluate was collected. The solution was condensed to a volume of approximately 1 mL utilizing a rotary evaporator. For the subsequent step, an acid alumina column was employed. The concentrated extract was transferred from the upper portion of the silica gel column to the acid alumina column. The column was subsequently eluted with 10 mL of hexane, followed by 30 mL of a dichloromethane/hexane mixture (2/98, v/v). The eluate was discarded. The column was subsequently subjected to elution using a 35 mL mixture of dichloromethane and hexane in a ratio of 40/60 (v/v). The eluate was collected and subsequently concentrated to a nearly dry state using a N2 stream.

For the analysis of PCDD/Fs, a high-resolution gas chromatograph/high-resolution mass spectrometer (HRGC/HRMS) was utilized. The HRGC (Hewlett-Packard 6970 Series Gas Chromatograph, California) was outfitted with an Rtx-5MS capillary column (Restek, PA). The column utilized in the experiment possessed a length of 30 m, an inner diameter of 0.25 mm and a film thickness of 0.25 µm. Additionally, a splitless injection method was utilized. The oven temperature program was established by utilizing predetermined parameters and employing He as the carrier gas. The high-resolution mass spectrometer employed in this research was the HRMS (Micromass Autospec Ultima, Manchester, UK). The instrument utilized a positive electron impact (EI+) source and functioned in the analyzer mode of selected ion monitoring with a resolving power of 10,000. The electron energy and source temperature were specified as 35 eV and 250°C, respectively.

 
2.4 Sampling and Analysis of Solid Specimens

The ash samples obtained for analysis were collected from either the air pollution control device units or the wastewater treatment system, specifically from the primary quenching tower ash, sludge, or fabric filter ash. The solid specimens underwent a drying process at a temperature of 105°C using a heating furnace. Subsequently, they were ground to a particle size smaller than 149 µm (passing through a mesh #100 sieve) in order to ensure sample homogeneity.

To analyze the metal contents, a 0.1-g portion of the ground sample was added to a carefully prepared acid mixture. This acid mixture comprised of 2 mL of 32% HCl, 4 mL of 67% HNO3, and 0.5 mL of 32% HF. The mixture was subsequently transferred into Teflon tubes and sealed tightly. The samples were digested using a microwave heater (CEM MARS 6) with the following a specific heating program. The temperature was gradually raised from room temperature to 180°C at a rate of 4°C min1, and then further increased from 180°C to 220°C at a rate of 3°C min1. The temperature was maintained at 220°C for 20 mins, followed by a cooling process facilitated by induced air. To ensure the removal of any residual hydrofluoric acid, the digested solutions underwent treatment with boric acid. We diluted the solutions with deionized water to achieve a final volume of 25 mL. Subsequently, we filtered the solutions using a mixed cellulose ester filter with an average pore size of 1 µm in order to eliminate any suspended solid. The metallic constituents in the digested solution were analyzed through the application of atomic absorption spectroscopy, employing the Agilent Technologies 55AA instrument.

 
3 RESULTS AND DISCUSSION


 
3.1 Analysis of Stack Flue Gas Pollution Characteristics

Fig. 2 illustrates the operational temperature of the units within the incineration system. At the outlet of the secondary incineration chamber, the operating temperature was maintained within the range of 900°C to 1,100°C. In cases where the operating temperature of the secondary incineration chamber fell below 900°C, the diesel burner was activated to raise the temperature of the flue gas. During the period when the operating temperature exceeded 1,100°C, the diesel burner was deactivated. The average temperature recorded during the sampling period was 1,009°C, which was maintained to facilitate the decomposition of unburned organic pollutants. The initial quenching procedure effectively decreased the temperature of the flue gas to an average of 309°C (with a range of 249°C to 369°C). After the temperature reduction achieved through the two-stage scrubbing process, the operating temperature of the flue gas in the subsequent units was approximately 70°C.

Fig. 2. Operating temperature of units in the incineration system.Fig. 2. Operating temperature of units in the incineration system.

Fig. 3 illustrates the relationship between the concentration of gas species and the duration of operation. The concentration of O2 was regulated to be above 12% in order to sustain an oxygen-rich environment. However, there were still disturbances observed, which could potentially be attributed to an uneven feeding rate of input materials. Due to the inherent limitations in achieving complete combustion of gas pollutants, an instantaneous peak concentration of CO at approximately 150 ppmv was observed. Table 1 presents the mean concentration of pollutants in the flue gas. The average concentrations of O2, CO2, and CO in the flue gas were measured to be 14.1%, 5.8%, and 37 ppmv, respectively. According to the established definition of combustion efficiency, the combustion efficiency was determined to be 99.94%. Furthermore, the average concentration of CO was only 37 ppmv, which was approximately one order of magnitude lower than the regulated standard. The analysis of flue gas indicated that the combustion process was maintained with an excess of oxygen to avoid incomplete combustion, leading to the efficient decomposition of MLW (Aurell and Marklund, 2009). The concentration of PCDD/Fs in the flue gas was measured to be 0.213 µg I-TEQ Nm–3 which was found to be only half of the regulated standard. This significant reduction can be attributed to the implementation of complete combustion techniques and the utilization of advanced APCDs.

Fig. 3. Concentration of gas species in the flue gas: (a) O2 and CO2, (b) CO, and (c) NOx and SO2.Fig. 3. Concentration of gas species in the flue gas: (a) O2 and CO2, (b) CO, and (c) NOx and SO2.

Table 1. Concentrations of gas components in the stack flue gas. 

The average concentration in the flue gas was measured at 57 ppmv. The generation of thermal NOx can be attributed to the elevated local temperature employed for the purpose of melting the bottom ash within the primary combustion chamber. The incineration system lacked a NOx treatment unit, such as selective catalytic reduction (SCR), for the removal of NOx (Fang et al., 2019; Liu et al., 2019). However, the NOx concentration was found to be only one-third of the permissible limit set by the emission standard. The reduction in the formation of NOx by 30.9% can be attributed to the achievement of novel flameless oxidation in the primary incineration chamber (Lin et al., 2021). The SO2 concentration was found to be an average of 1.0 ppmv, which can be attributed to the relatively low S content present in the feed material.

 
3.2 Analysis of Pollutant Concentration at Various Sampling Locations

Table 2 presents the concentrations of acid pollutants at various sampling locations. At location A, the primary quenching tower effectively eliminated the acid gas, leading to concentrations of HCl, HNO3, and H2SO4 at 552.0, 18.4, and 73.5 mg Nm3, respectively. The original HCl concentration exceeded the regulated standard by a factor of five (TME, 2006e). After undergoing the two-stage scrubbing process and dewatering unit, the concentrations of HCl, HNO3, and H2SO4 were reduced to 47.9, 0.999, and 3.51 mg Nm3, respectively. At location C, the concentrations of these compounds exhibited a further decrease to 0.052, 0.002, and 0.053 mg Nm3, respectively. This phenomenon can be attributed to the effectiveness of the fabric filter in removing fine particulate matter that may have absorbed wet acid gas. The HCl concentration was 3 orders of magnitude lower than the regulated standard. The removal efficiencies for HCl, HNO3, and H2SO4 were found to be 99.99%, 99.97%, and 99.93%, respectively. The successful removal of acid in this study provided effective protection against acid corrosion for the APCD units (Kuo et al., 2020).

Table 2. Concentrations of acid pollutants in the flue gas at different sampling locations.

Table 3 displays the concentrations of VOCs and major species in the flue gas at various sampling locations. The concentrations of TNHMCs in the flue gas at locations A, B, and C were measured to be 66.4, 37.5, and 22.6 ppmv, respectively. These TNHMCs could potentially have originated from the quenching water, rather than being a byproduct of the incineration process. This is because the quenching water used in the process was recycled from treated liquid laboratory waste, which may have contained organic materials. The wet scrubber and fabric filter demonstrated an overall removal efficiency of 66.0% for TNMHCs was 66.0%. The predominant VOC species identified in the study were ethanol (6,521 ppbv), 2-propanol (1,818 ppbv), acetone (872 ppbv), and methylene chloride (152 ppbv). These compounds are frequently employed in laboratory experiments. The removal efficiencies of these VOC species varied between 51.3% and 84.1% due to their high water solubility.

Table 3. Concentrations of VOC species in the flue gas at different sampling locations.

 
3.3 Analysis of Particulate Matter Characteristics in Flue Gas

Table 4 presents the recorded concentrations of FPM and CPM at three designated sampling locations. At location A, the concentrations of FPM and CPM were measured to be 1,770 and 4,390 mg Nm3, respectively. It was found that CPM accounted for 71.3% in the total particulate matter (TPM) mass. The results indicate that the interaction between the injected NaOH solution and acid pollutants, primarily HCl, resulted in a significant generation of CPM. After undergoing a two-stage scrubbing process, a significant reduction of 97.8% in TPM, 92.9% in FPM, and 99.8% in CPM was achieved. The phenomenon can be elucidated in the following manner. The flue gas temperature at locations A and B was approximately 450°C and 70°C, respectively. After being discharged from the initial quenching tower, the particulate matter primarily existed in the form of CPM. During the two-stage scrubbing process, these particles were condensed into FPM as they were cooled to approximately 70°C. The FPM was subsequently dissolved or incorporated into the scrubbing wastewater. Therefore, at location B, FPM had a high level of 126 mg Nm3 and constituted the majority (94.0%) of the remaining TPM.

Table 4. Characteristics of particulates at different sampling locations.

After passing through the fabric filter, the concentrations of FPM and CPM were reduced to 4 and 3 mg Nm3, respectively. The overall efficiencies of APCDs on the FPM, CPM, TPM were found to be 99.8%, 99.93%, and 99.89%, respectively. The preceding investigation also documented that the enhanced APCD system exhibited superior efficacy in the elimination of fine particulate matter (Lin et al., 2022). The operational procedure of this sophisticated APCD system aligns with the developmental trajectory discussed in a prior investigation (Feng et al., 2018).

Table 5 presents the quantitative data regarding the metal composition and PCDD/F content found in solid specimens. The levels of PCDD/Fs in solid specimens collected at locations A, B, and C were measured to be 0.347, 90.1, and 0.985 ng I-TEQ g1, respectively. The input materials used in this study were MLW which typically have high levels of Cl (approximate 10%) (Załuska et al., 2022). The presence of Cl was identified as a significant contributing factor in the formation of PCDD/Fs (Hatanaka et al., 2005). The operating temperature of air pollution control devices (APCDs) varied between 400°C and 600°C, which aligns with the de nova temperature range conducive to the formation of PCDD/Fs as reported by Cai et al. (2022). Consequently, the concentrations of PCDD/Fs in fly ashes were found to be considerably higher compared to those detected in municipal solid waste incinerators (Gidarakos et al., 2009; Lin et al., 2022). In the present system, the removal of PCDD/Fs primarily occurs during the two-stage scrubbing process. The wastewater that underwent treatment was reintroduced into the two-stage scrubbing process, resulting in the accumulation of pollutants in the wastewater. Therefore, the sludge that was removed from the wastewater exhibited a significantly higher concentration of PCDD/F compared to the other two fly ashes.

Table 5. Composition of metals, PCDD/Fs, and Cl in fly ashes and sludge.

The primary quenching tower ash was found to contain significant amounts of Na (192,000 mg kg1) and Cl (289,000 mg kg1), while other metals were present in trace amounts (< 1,000 mg kg1). The NaCl salt primarily originates from the reaction (as demonstrated in Eq. (1)) between the injected NaOH solution and HCl.

 

The sludge exhibited low concentrations of Na and Cl, with values of 9,700 and 10,200 mg kg1, respectively. Additionally, the levels of other metals in the sludge were below 1,000 mg kg1. The concentrations of Na and Cl in the fabric filter ash were measured at 30,100 and 10,200 mg kg1, respectively. Additionally, the levels of other metals present in the ash were found to be below 1,000 mg kg1. The analysis indicates that the predominant metallic components found in the particulate matter were primarily Na and Cl which were determined to be non-hazardous. However, the levels of PCDD/F in these solid specimens exceeded the regulated standard and were classified as hazardous materials. Consequently, they were subjected to further treatment in a plasma melting system (Kuo et al., 2010).

 
4 CONCLUSIONS


This study aims to investigate the emission characteristics associated with the incineration of MLW, utilizing an advanced scrubbing/filtration system. By integrating waste solvent injection into the primary combustion chamber, the combustion efficiency was enhanced to 99.93%. The advanced APCDs showcased exceptional efficacy in the elimination of pollutants, effectively reducing their concentrations to levels significantly below the regulated standards. The two-stage scrubbing process demonstrated effective protection of the fabric filter against acid gas corrosion, specifically HCl. Additionally, the aforementioned procedure involved the conversion of CPM into FPM through the reduction of the flue gas temperature to 70°C. The fabric filter demonstrated efficient removal of FPM through a sequential filtration process. Consequently, the implementation of state-of-the-art scrubbing and filtration technology has shown remarkable effectiveness in eliminating FPM and CPM, achieving removal rates of 99.8% and 99.93% respectively. These results surpass the performance of conventional APCDs.


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