Study on Low-Temperature Pyrolysis of Dioxins in Municipal Solid Waste Incineration Fly Ash Using Water-Washed Synergistic Catalysts

Abstract

Fly ash produced by Municipal Solid Waste Incineration (MSWI) contains significant quantities of dioxins, posing a major challenge for safe disposal. Compared to other high-temperature disposal methods, low-temperature pyrolysis (<500 °C) can efficiently degrade dioxins in fly ash at relatively low temperatures. To better understand the effects of water-washing and catalysts on dioxin decomposition during low-temperature pyrolysis, this study investigates the impact of water-washing and three different catalysts (V₂O₅-WO₃/TiO₂, Fe/C, and CaO) on the decomposition of dioxins in washed fly ash (WFA). The results indicate that, despite the fact that water-washing pretreatment causes dioxin enrichment in WFA, the toxic equivalent quantity (TEQ) of dioxins within WFA remains lower at the identical pyrolysis temperature when contrasted with that in raw fly ash (RFA). Low-temperature pyrolysis carried out at 250 °C is capable of degrading 99.3% of the dioxins present in water-washed fly ash, achieving a significantly better performance compared to the raw fly ash, which has a degradation efficiency of merely 80%. Nevertheless, when the temperature is set at 210 °C, the degradation efficiency of the WFA turns out to be relatively low, only reaching 29%. The addition of catalysts remarkably promoted dioxin degradation at 210 °C. Among them, CaO exhibited the most outstanding performance, achieving a degradation efficiency as high as 94.8%. It should be emphasized that the catalyst ratio plays a pivotal role in the degradation process. Specifically, the proportion of CaO should not to be less than 10 wt.%.

1. Introduction

Municipal solid waste incineration fly ash has been classified as hazardous waste due to its high concentrations of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs), heavy metals (such as Cd, Cr, Cu, Ni, Pb, and Zn), and soluble chlorides [1,2]. The effective and safe disposal of fly ash has become an urgent challenge. China’s Ministry of Ecology and Environment stated in August 2020 that fly ash can be repurposed as an alternative material in specific construction after effective, harmless disposal [3]. Low-temperature pyrolysis, initially proposed by Hagenmaier et al. in 1987 [4], can effectively reduce the de novo synthesis of dioxins under oxygen-deficient conditions at 250–400 °C. Compared with other thermal treatments, low-temperature pyrolysis can achieve higher dioxin degradation efficiency, consume less energy, and be more effective than non-thermal disposal [5,6]. This approach not only enables the efficient degradation of dioxins, but also facilitates the large-scale disposal of fly ash, making it a promising technology for the further resource utilization of fly ash. It is worth emphasizing that the temperature ranges for low-temperature thermal degradation and the regeneration of PCDD/Fs overlap considerably, making it essential to mitigate the risks associated with dioxin reformation. However, research on the interplay between PCDD/Fs degradation and regeneration remains limited, particularly in identifying effective detoxification strategies under low-temperature conditions [7].

Low-temperature pyrolysis in fly ash under an inert atmosphere primarily proceeds through dechlorination/hydrogenation reactions [8]. Weber et al. [9] proposed a binary principle regarding the formation and destruction of PCDD/Fs: during the degradation process, both dioxin formation and destruction occur simultaneously. Current research on low-temperature thermal degradation technology primarily focuses on optimizing process parameters, such as reaction temperature, reaction time, and reaction atmosphere. Under a nitrogen atmosphere, thermal treatment at 350 °C for 1 h achieved a PCDD/Fs degradation efficiency of 99.7% [10]. When 1% oxygen was present, the degradation efficiency remained high at 95.8% after heating at 400 °C for 90 min [11]. In an oxygen-containing atmosphere, within the temperature range of 300 °C to 500 °C, heterogeneous reactions among carbon (C), oxygen (O₂), and chlorine (Cl) occur under the catalytic effects of Cu and Fe, leading to the de novo synthesis of dioxins [12]. Therefore, low-temperature pyrolysis in an inert atmosphere effectively prevents the issue of de novo synthesis of dioxins [13]. Chlorine is a key element in the synthesis of dioxins, and it plays a crucial role in the interconversion of PCDD/Fs [14,15]. In low temperatures, dioxins can form in fly ash via precursors, such as chlorophenols or chlorobenzenes. Trinh et al. [16] demonstrated that washing fly ash (WFA) with a liquid-to-solid ratio of 3:1 for 5 min could reduce chlorine content from 23.4% to 7.1%. Their study on the low-temperature pyrolysis of WFA revealed that the removal efficiency of PCDD/Fs was significantly higher in the range of 200–300 °C.

Low-temperature pyrolysis can effectively improve degradation efficiency or reduce treatment temperatures by adding catalysts. Commercial V₂O₅-WO₃/TiO₂ (VW/Ti), Fe/C, and other single-metal (Mo, Mn, and Ce) catalysts are the representative catalysts [17,18,19,20,21]. Trinh et al. [16] investigated the effect of the precious metal catalyst Pd by adding 10% Pd/γ-Al₂O₃ or Pd/C to fly ash at 350 °C for 15 min, resulting in an increase in degradation efficiency from 30.7% to 98.3%. Boos et al. [22] found that via catalytic oxidation the statutory emission level of 0.1 ng TEQ/m³ could be achieved. Additionally, Trinh et al. [23] investigated the catalytic effects of Fe/C, Co/C, Ni/C, and Cu/C transition metals on the pyrolysis of dioxins in fly ash, with a degradation efficiency of 75.0%, 81.2%, 94.5%, and 88.3%, respectively. These results show that the addition of these catalysts can significantly enhance the efficiency of low-temperature pyrolysis.

In this study, the physicochemical characteristics, dioxin concentrations, and the impact of water-washing on the low-temperature pyrolysis of dioxins were investigated. Additionally, the degradation effects of different catalysts, such as VW/Ti, Fe/C, and CaO catalysts, were evaluated, including the role of the CaO ratio. The findings provide insights into the low-temperature pyrolysis of fly ash.

2. Results and Discussion

2.1. Characteristics of Water-Washed Fly Ash

2.1.1. Physicochemical Properties of Water-Washed Fly Ash

Table 1. Main elemental composition of RFA and WFA (wt.%).

Element	Ca	Cl	Na	K	Si	S	Fe	Mg	Al	Zn	F	P	Pb	LOI *
RFA	25.94	21.14	9.71	4.29	4.05	2.27	1.51	1.34	1.25	0.73	0.40	0.35	0.25	25.6%
WFA	34.14	0.62	0.91	0.61	9.07	2.10	3.43	3.43	2.97	1.36	2.38	0.85	0.23	18.9%

* LOI, loss on ignition.

presents the main elemental composition of RFA and WFA. Compared with RFA, the concentrations of Cl, Na, and K in WFA were significantly reduced. During the washing process, soluble chlorides in the fly ash were substantially removed, with the Cl content dropping from 21.14 wt% to 0.624 wt%, achieving a removal rate of 97%. In contrast, elements that were water insoluble, such as Si, Fe, Al, and Mg, showed an increase due to the reduction in fly ash mass.

After washing, the mass of the fly ash decreased from 50.0 g to 30.59 g, consistent with previous findings [1]. The surface morphology of RFA and WFA is illustrated in Figure 1. The RFA (a,c) showed spherical, relatively uniform particles with a porous, loose structure, and hexagonal plate-like substances on the surface. This morphological change was likely due to the incineration temperature range of the grate furnace (900–950 °C), which allowed lower-boiling-point chlorides to vaporize and condense in the flue gas, forming spherical, small particles with cubic crystal lattices of chloride salts. In contrast, the WFA (b,d) had irregularly shaped particles with fewer surface pores. After washing and drying, the particles tended to agglomerate, forming larger and more irregular structures, with CaClOH converting to Ca(OH)₂ following the water-washing process [24].

2.1.2. The Migration of Dioxins During the Water-Washing Process

The toxic equivalent concentrations of 17 dioxin congeners in RFA, WFA, and waste water are shown in Figure 2. The dioxins in RFA were 427.24 ng I-TEQ/kg. Due to the low solubility of dioxins in water, the dioxin in WFA increased to 695.44 ng I-TEQ/kg, representing a 62.77% increase. The distribution of dioxin congeners in RFA and WFA was similar, with an increase to the mass loss of the fly ash, indicating dioxin enrichment in the WFA. The dioxin concentration in the fly ash washing water was 61.95 ng I-TEQ/m³, accounting for only 0.145% of the concentration in the RFA. However, this value was twice the standard for drinking water (30 ng I-TEQ/m³), which was necessary for the treatment of the washing solution prior to discharge. Fly ash washing waste water contains trace dioxins, heavy metals, and ions such as chlorine, potassium, sodium, calcium, and magnesium, along with suspended solids. To facilitate the resource recovery of potassium and sodium salts, pretreatment is necessary to remove the calcium, magnesium, and suspended solids. This is achieved through chemical precipitation, neutralization, and filtration, followed by mechanical vapor recompression for salt recovery.

2.2. Degradation Characteristics of Dioxins in WFA

2.2.1. Effect of Water-Washing Treatment on Low-Temperature Pyrolysis

This section primarily analyzed the impact of the water-washing treatment on the low-temperature pyrolysis. The toxic equivalent concentrations of dioxins in RFA and WFA after low-temperature pyrolysis are shown in Figure 3. Although water-washing treatment led to dioxin enrichment in WFA [25,26], the TEQ of dioxins in WFA was lower at the same pyrolysis temperature compared to RFA. At 200 °C (A-1) and 250 °C (A-2), the dioxins in RFA were effectively degraded, achieving degradation efficiencies of 27.1% and 80.0%, respectively. For WFA, a significant improvement was observed, with degradation efficiencies of 56.1% at 200 °C (B-1) and 99.3% at 250 °C (B-4). After 10 min of treatment at 250 °C, the dioxin TEQ in WFA decreased to 4.85 ng I-TEQ/kg, well below the European standard of 20 ng I-TEQ/kg.

2.2.2. Effect of Reaction Temperature on Degradation Characteristics of Dioxins in WFA

The distribution of toxic equivalent (TEQ) concentrations for 17 dioxin congeners in fly ash after pyrolysis at 200 °C, 210 °C, 220 °C, and 250 °C are shown in

Table 2. Distribution of dioxin congener concentrations in WFA at different temperatures.

Compound	Isomer	TEF	Temperatures (°C)
			0	200	210	220	250
T4CDD	2378	1	45.03	86.92	205.05	5.83	0.66
P5CDD	12378	0.5	102.02	75.24	119.76	2.91	0.58
H6CDD	123478	0.1	13.45	4.95	5.38	0.20	0.12
H6CDD	123678	0.1	39.74	10.53	12.40	0.44	0.86
H6CDD	123789	0.1	22.06	11.16	13.41	0.41	0.59
H7CDD	1234678	0.01	33.06	4.00	3.58	0.15	0.45
O8CDD	12346789	0.001	9.46	0.43	0.098	0.0063	0.029
T4CDF	2378	0.1	35.41	16.52	34.11	0.76	0.066
P5CDF	12378	0.05	29.51	10.62	16.11	0.41	0.060
P5CDF	23478	0.5	150.17	42.90	43.96	0.90	0.87
H6CDF	123478	0.1	60.64	12.80	10.78	0.40	0.10
H6CDF	123678	0.1	66.79	15.83	15.58	0.50	0.14
H6CDF	234678	0.1	10.24	1.53	1.42	0.081	0.044
H6CDF	123789	0.1	58.06	9.61	8.35	0.35	0.20
H7CDF	1234678	0.01	16.44	2.13	1.65	0.083	0.038
H7CDF	1234789	0.01	3.30	0.34	0.31	0.020	0.013
O8CDF	12346789	0.001	0.077	0.055	0.012	0.00035	0.00072
TOTAL (ng I-TEQ/kg)			695.51	305.62	492.01	13.51	4.85

. At 200 °C, the TEQ concentration decreases by over 50%. Notably, the degradation efficiency of OCDD and 1,2,3,4,6,7,8-HCDD is high, at 95.45% and 87.9%, respectively. However, the concentration of 2,3,7,8-TCDD increases by 93%, which can be attributed to the dechlorination of highly chlorinated dioxins into lower-chlorinated forms like 2,3,7,8-TCDD at 200 °C. After low-temperature pyrolysis, the concentration of low-chlorinated dioxins in the fly ash remains relatively high [26]. At 200 °C, the concentration of PCDFs in the water-washed fly ash (WFA) decreases, indicating that lower-chlorinated PCDFs are more susceptible to dechlorination than lower-chlorinated PCDDs.

When the reaction temperature was increased to 220 °C, dioxins in the fly ash rapidly degraded, indicating that decomposition reactions dominate in WFA at this temperature. When the reaction temperature was increased to 250 °C, dioxins in the fly ash underwent further decomposition. The concentration of each dioxin congener tended to stabilize, suggesting that low-temperature pyrolysis achieved near-complete degradation, with an overall dioxin degradation rate exceeding 99%. This phenomenon is likely due to the simultaneous chlorination and dechlorination of dioxins in the fly ash, influenced by the presence of CuCl₂, as shown in reaction Equations (1) and (2). Copper chloride is a relatively strong oxidizing agent and can provide chlorine atoms (Cl) to participate in chemical reactions. When interacting with substances containing an aromatic hydrocarbon structure, such as C₁₂H₈O₂, the Cl atoms from copper chloride attack specific hydrogen positions on the benzene ring, leading to an electrophilic substitution reaction. This process accelerates the regeneration of dioxins. Since CuCl₂ was water-soluble, water-washing treatment significantly reduced the Cl content in fly ash, thereby greatly reducing dioxin regeneration. This enhanced dechlorination in WFA was likely due to the reduced chlorine content, which lowered the CuCl₂ catalyst levels in the fly ash, thereby weakening the chlorination reactions of PCDD/Fs [27,28].

C₁₂H₈O₂/C₁₂H₈O + CuCl₂ ↔ C₁₂H₇O₂Cl/C₁₂H₇OCl + 2CuCl + HCl

(1)

CuCl + Cl⁻ ↔ CuCl₂

(2)

2.2.3. Analysis of Degradation Characteristics of 136 Dioxins

The distribution of 136 dioxin congeners in WFA after pyrolysis at 200 °C, 210 °C, 220 °C, and 250 °C is shown in

Table 3. Dioxin concentrations at different temperatures.

Compound	Temperatures (°C)
	200	210	220	250
PCDDs (ng/kg)	6032.59	4183.39	282.34	31.09
PCDFs (ng/kg)	7064.22	5251.16	312.92	52.54
PCDD/Fs (ng/kg)	13,096.81	9434.55	595.27	83.63
PCDDs/PCDFs	0.85	0.79	0.90	0.59
Cl-PCDD	4.85	4.99	5.21	5.78
Cl-PCDF	4.85	4.57	4.73	5.51
TEQ (ng I-TEQ/kg)	305.62	492.01	13.51	4.85

. As the reaction temperature increased, the concentrations of PCDDs and PCDFs gradually decreased. Above 220 °C, the dioxin concentration dropped sharply, and the degradation efficiency positively correlated with temperature. Both the chlorination degree of PCDDs and PCDFs showed an increasing trend, and PCDDs were more significantly affected by temperature changes.

The percentage distribution of 136 dioxin congeners in WFA at 200 °C, 210 °C, 220 °C, and 250 °C is shown in Figure 4. At 200 °C, the dominant congeners in the fly ash were TCDD and PeCDD, which account for 29% and 31%, respectively. The distribution of PCDD congeners was ranked as TCDD > PeCDD > HxCDD > HpCDD > OCDD, while the distribution of PCDF congeners was ranked as TCDF > PeCDF > HxCDF > HpCDF > OCDF. At 210 °C, the proportion of TCDD and TCDF increased, as higher-chlorinated dioxins underwent dechlorination to lower-chlorinated dioxins. At 220 °C, the percentages of TCDD and TCDF declined. However, they still remained dominant, accounting for 40% and 55% respectively. Lower-chlorinated at 210 °C, the proportion of TCDD and TCDF rose. This was because higher-chlorinated dioxins underwent dechlorination to transform into lower-chlorinated dioxins. At 250 °C, the dominant congeners in the fly ash were HpCDD and HxCDF, which accounted for 29% and 38%, respectively. The distribution of PCDD congeners was ranked as HpCDD > HxCDD > PeCDD > TCDD > OCDD, and the distribution of PCDF congeners was ranked as TCDF > PeCDF > HxCDF > HpCDF > OCDF. This further confirmed that as the temperature increased, the degradation efficiency of lower-chlorinated dioxins increased accordingly [29].

2.3. Low-Temperature Catalytic Pyrolysis of Dioxins

2.3.1. The Degradation Effects of Different Catalysts on Dioxins

From the previous experiments, it was observed that the dioxin degradation efficiency in WFA at 200 °C and 210 °C was relatively poor. To improve the degradation efficiency, three different catalysts (Fe/C, VWTi, and CaO catalysts) were added to WFA to investigate. The dioxin concentrations after the low-temperature pyrolysis of WFA with different catalysts are shown in

Table 4. Dioxin concentrations in WFA with different catalysts.

Compound	Fe/C		VWTi		CaO
	200 °C	210 °C	200 °C	210 °C	200 °C	210 °C
PCDDs (ng/kg)	5295.06	1324.21	2995.02	520.95	2749.04	232.36
PCDFs (ng/kg)	6428.02	2004.71	2858.97	559.41	2532.74	336.97
PCDD/Fs	11,723.09	3328.91	5853.99	1080.35	5281.78	569.33
PCDDs/PCDFs	0.82	0.66	1.05	0.93	1.09	0.69
Cl-PCDD	5.31	5.15	5.27	5.22	5.12	5.14
Cl-PCDF	4.76	4.68	4.76	4.77	4.73	4.80
TEQ (ng I-TEQ/kg)	578.82	145.29	316.39	54.42	346.91	25.67

. It is clear that at 200 °C, all three catalysts promoted the low-temperature pyrolysis of dioxins, resulting in a reduction in the total dioxin concentration. Compared to the total concentration of 136 dioxin congeners in WFA at 200 °C (in Table 4), the dioxin concentrations decreased by 10.49%, 55.30%, and 59.67%, respectively.

When the reaction temperature was raised to 210 °C, the catalytic activity of all three catalysts was significantly enhanced, improving the degradation efficiency of dioxins in low-temperature pyrolysis. The dioxin toxic equivalent concentration degradation efficiencies were 70.47%, 88.94%, and 94.78%, respectively. The CaO catalyst achieved the best degradation, with the toxic equivalent concentration of the fly ash reaching 25.67 ng I-TEQ/kg, which is comparable to the degradation efficiency at 220 °C in WFA. This indicates that the CaO catalyst can further lower the reaction temperature for low-temperature pyrolysis. CaO can adsorb and destroy the precursors of PCDD/Fs to achieve inhibition [6]. On the other hand, the chlorination degree of PCDDs and PCDFs decreased in the presence of the Fe/C catalyst at 210 °C. This indicates that the Fe/C catalyst has a strong catalytic activity for the degradation of higher-chlorinated dioxins. In contrast, the VWTi and CaO catalysts showed little effect on the chlorination degree of dioxins.

The percentage distribution of 136 dioxin congeners in WFA with different catalysts is shown in Figure 5. The distribution of dioxin congeners in the fly ash was similar. TCDD and TCDF became the dominant congeners. The distribution of PCDD congeners was ranked as TCDD > PeCDD > HxCDD > HpCDD > OCDD, while the distribution of PCDF congeners was ranked as TCDF > PeCDF > HxCDF > HpCDF > OCDF. This further confirmed that dechlorination was the main reaction in fly ash at 210 °C, and the dechlorination efficiency of TCDD was relatively slow. The presence of catalysts accelerated the dechlorination efficiency of lower-chlorinated dioxins [30,31].

2.3.2. Effect of CaO Catalyst Addition

To further investigate the effect of the CaO catalyst, different mass ratios of 0 wt.%, 1 wt.%, 5 wt.%, and 10 wt.% were chosen for study in the fly ash. The results in Figure 6 showed that the dioxin toxic equivalent concentrations in the fly ash were 492.01 ng I-TEQ/kg, 165.55 ng I-TEQ/kg, 110.93 ng I-TEQ/kg, and 25.67 ng I-TEQ/kg, respectively. The corresponding dioxin degradation efficiencies were 29.26%, 76.19%, 84.05%, and 96.31%. It was evident that as the CaO ratio increases, the toxic equivalent concentration of dioxins in the fly ash decreases, indicating that CaO promotes the dechlorination process. When the CaO ratio was 10 wt.%, the concentrations of various dioxin congeners were significantly reduced. Therefore, the CaO ratio is required to be no less than 10 wt.%.

In addition, it was discovered that the concentration of 2,3,7,8-TCDD and OCDF was 1.5 and 5 times higher than WFA when the addition of CaO was 1 wt.%, respectively. In contrast, the concentrations of other dioxins significantly decreased. The increase in the 2,3,7,8-TCDD concentration was related to the dechlorination of other higher-chlorinated congeners, while the increase in OCDF was likely due to the formation of precursor compounds. During the pyrolysis process of WFA, higher-chlorinated dioxins underwent dechlorination and transformed into lower-chlorinated dioxins. The added CaO reacted with HCl to reduce the chloride, thereby promoting the dechlorination reaction [6].

When 5 wt.% CaO was added, the concentration of OCDD remained almost unchanged compared to the untreated WFA, while the concentration of OCDF increased by 18 times. The increase in the OCDF concentration may be due to the condensation reaction of precursor compounds. CaO promoted the condensation of precursor compounds by an acid-base neutralization reaction. However, since the dechlorination of dioxins and the condensation of precursors occurred simultaneously, the CaO catalyst was gradually consumed, leading to a slower dechlorination rate of PCDFs. This further confirmed that the formation rate of PCDFs exceeded the degradation efficiency at low CaO concentrations. Therefore, it is essential to ensure an adequate amount of catalysts to sustain the dechlorination of higher-chlorinated dioxins.

3. Materials and Methods

3.1. Materials

The fly ash used in this study was collected from the baghouse dust collector of a grate-type waste incineration plant in Huzhou, Zhejiang Province. The raw fly ash (RFA) was sieved and evenly divided into five portions to ensure accuracy and consistency of results. The CaO catalyst was purchased from Aladdin, while the VW/Ti catalyst was obtained from Shanghai Hanyu Environmental Protection Materials Co., Ltd. (Shanghai, China).

The Fe/C catalyst was prepared in the laboratory as follows. Initially, 14.5 g of Fe(NO₃)₃·9H₂O was dissolved in deionized water, and 20.0 g of activated carbon was added to the solution. The mixture was stirred at 200 rpm on a magnetic stirrer at 80 °C until it formed a uniform paste. The paste was then dried at 105 °C in an oven for 24 h to eliminate moisture. Subsequently, the dried material was thermally activated in a tube furnace under a nitrogen flow rate of 1.0 L/min. The temperature was increased to 150 °C and maintained for 15 min before being raised to 500 °C for an additional 2 h to complete the activation. After cooling, the catalyst was ground and sieved to achieve a particle size of 20–40 mesh, and the final product was stored in a cool, dry environment for subsequent applications. This process resulted in a catalyst with a targeted 10 wt.% iron content.

3.2. Experiment Design

In this study, a laboratory-scale low-temperature pyrolysis experimental setup for fly ash was designed and constructed, as shown in Figure 7. The system is equipped with heating, temperature control, and temperature monitoring capabilities. A lead-type armored thermocouple serves as the temperature sensor, with its probe positioned at the upper section of the quartz boat inside the pyrolysis reactor to accurately measure the temperature of the fly ash.

Prior to each batch of experiments, nitrogen (N₂) is introduced into the tube furnace at a flow rate of 200 mL/min and a temperature of 400 °C to purge the reactor and quartz boat for 30 min. This procedure ensures the complete removal of residual O₂ and dioxins from the system. After the experiment, fly ash samples are collected, along with XAD-II resin, toluene solution, and rinsing liquids from the reactor walls and quartz boat (washed with toluene). These samples are then stored in a sealed environment for further analysis and processing.

3.3. Water-Washing Treatment

The water-washing procedure was as follows. Initially, the fly ash was dried at 105 °C in an oven for 24 h to remove moisture. The dried material was then subjected to water-washing using deionized water, maintaining a liquid-to-solid ratio of 10 mL/g. The washing process was carried out on a constant temperature magnetic stirrer with a water bath set at 25 °C, stirring at a rotor speed of 200 rpm for 30 min to facilitate efficient washing. Following this, solid–liquid separation was performed through vacuum filtration using a 0.45 μm microporous filter membrane. The filtrate was collected, and the residue (washed fly ash) was dried again at 105 °C for 24 h to achieve a stable state. Finally, the dried fly ash was ground and sieved to obtain a uniform particle size, ensuring its suitability for subsequent applications.

3.4. Experimental Conditions

The experimental conditions are shown in

Table 5. Low-temperature pyrolysis experimental conditions.

Condition	Reactant	Temperature/°C	Time/mins	Atmosphere	Flow Rate mL/min
A-1	RFA	200	10	N2	200
A-2	RFA	250	10	N2	200
B-1	WFA	200	10	N2	200
B-2	WFA	210	10	N2	200
B-3	WFA	220	10	N2	200
B-4	WFA	250	10	N2	200
C-1	WFA + 10 wt.% Fe/C	200	10	N2	200
C-2	WFA + 10 wt.% VWTi	200	10	N2	200
C-3	WFA + 10 wt.% CaO	200	10	N2	200
C-4	WFA + 10 wt.% Fe/C	210	10	N2	200
C-5	WFA + 10 wt.% VWTi	210	10	N2	200
C-6	WFA + 10 wt.% CaO	210	10	N2	200
D-1	WFA + 5 wt.% CaO	210	10	N2	200
D-2	WFA + 1 wt.% CaO	210	10	N2	200

. Condition A and Condition B investigate the effects of water-washing treatment and different temperatures on the low-temperature degradation of dioxins in fly ash. Condition C and Condition D explore the impact of different catalysts and CaO ratios on the low-temperature degradation of dioxins in fly ash at a relatively lower temperature.

3.5. Analytical Methods

The primary elements in the fly ash were analyzed using an X-ray fluorescence spectrometer (XRF,ARL ADVANT’X IntelliPowerTM 4200, Thermo Fisher Scientific, Waltham, MA, USA). The surface morphology of the fly ash was examined using a scanning electron microscope (SEM, Sigma 300, Zeiss, Germany). The soluble chlorine content in the fly ash was measured using ion chromatography (IC, Integrion, Thermo Fisher Scientific, Waltham, MA, USA). Dioxin analysis was conducted with high-resolution gas chromatography-mass spectrometry (HRGC/MS, JMS-800D, JEOL Co., Tokyo, Japan) [32].

4. Conclusions

Despite the fact that water-washing pretreatment causes dioxin enrichment in WFA, the toxic equivalent quantity of dioxins within WFA remains lower at the identical pyrolysis temperature when contrasted with that in RFA. Low-temperature pyrolysis carried out at 250 °C is capable of degrading 99.3% of the dioxins present in water-washed fly ash, achieving a significantly better performance compared to the raw fly ash, which has a degradation efficiency of merely 80%. Nevertheless, when the temperature is set at 210 °C, the degradation efficiency of the WFA turns out to be relatively low, only reaching 29%. The addition of catalysts remarkably promoted dioxin degradation at 210 °C. Among them, CaO exhibited the most outstanding performance, achieving a degradation efficiency as high as 94.8%. It should be emphasized that the catalyst ratio plays a pivotal role in the degradation process. Specifically, the proportion of CaO should not to be less than 10 wt.%. Furthermore, it is worth noting that the dioxin in the fly ash waste water is 61.95 ng I-TEQ/m³, which is only 0.145 % of the raw fly ash, but it is twice the limit for drinking water standards (30 ng I-TEQ/m³), indicating that the waste water must be treated before discharge.