Effect of Potassium Ferrate as a Dewatering Conditioner on Sludge Pyrolysis Characteristics and the Releasing Characteristics of Nitrogen, Sulfur, and Chlorine during Sewage Sludge Pyrolysis

Abstract

Sludge pyrolysis is a promising method for treating excess sludge as a by-product of municipal sewage plants, allowing for energy self-sufficiency and resource recovery. Before sludge pyrolysis begins, a few conditioning agents are added to the sludge that promote sludge dewatering. Potassium ferrate (K₂FeO₄) is applied as a conditioning agent with both cracking and flocculation effects, but the effects of K₂FeO₄ on the release characteristics of nitrogen, sulfur, and chlorine during sludge pyrolysis have not been elucidated. In this study, we analyzed the sludge pyrolysis characteristics and chemical state changes of N, S, and Cl contaminants in the dewatered sludge after K₂FeO₄ conditioning before and after pyrolysis. Further, the release characteristics of condensable/noncondensable gases during pyrolysis were assessed using thermogravimetric mass spectrometry (TG-MS) and pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS) analyses before and after conditioning. We found that potassium pertechnetate reduced the activation energy required for the sludge in the pyrolysis process. Noticeably this process made the sludge more susceptible to thermal decomposition leading to volatile production and also influenced the release of different contaminants generated by the pyrolysis process. Moreover, K₂FeO₄ promoted the release of C/H/O gases and reduced the release of N/S/Cl pollutant gases from the sludge. Overall, this study provides a theoretical basis for the selection of conditioning agents for the sludge conditioning and dewatering steps during the sludge pyrolysis process.

1. Introduction

Currently, a majority of municipal sewage treatment plants use the activated sludge method to treat sewage. The treatment process generates a large amount of sludge by-products with a complex composition, which is not only rich in nutrients such as nitrogen and phosphorus, but also contains many harmful substances such as heavy metals, pest eggs, and fungi [1,2]. With the intensification of urbanization, the annual sludge production in China is expected to exceed 9 × 10⁷ t by 2025, and the large amount of sludge produced has led to huge environmental problems [3]. If the sludge is not disposed of properly, it increases the risk of secondary environmental pollution [4].

The common methods of sludge disposal are landfilling, composting, incineration, and pyrolysis [5,6]. Landfilling of sludge carries the risk of contaminating groundwater, occupies a lot of site space, and has a low utilization rate [7]. While sludge composting is an effective method for the utilization of nitrogen and phosphorus nutrients, it can contaminate soil with heavy metals. Zhang, et al. [8] found that the potential ecological risk of heavy metal mercury in compost is very high, and long-term sludge composting will have a cumulative adverse environmental impact. Incineration can markedly reduce the amount of sludge, but flue gas treatment is more difficult. Pyrolysis is currently the most promising treatment method, which produces biomass oil, gas, and char, all of which can be used [9,10]. Thus, the maximum resource utilization of sludge with the least environmental harm can be achieved using pyrolysis technology [11,12]. The water content of sludge is up to 99%, which affects its transportation and subsequent resource utilization [13]. Therefore, sludge dewatering plays an important role in its treatment and disposal.

Iron salts are a common class of sludge dewatering conditioners that are added to improve the sludge dewatering performance. Among the iron salts, potassium ferrate (K₂FeO₄) is widely used in sludge conditioning and dewatering because of its strong oxidizing and flocculating effects [14]. The hydrolysis of K₂FeO₄ produces Fe(OH)₃, which makes sludge flocculate into agglomerates, reduces the stability of sludge, improves the settling performance, and thus increases the dewatering capacity of sludge [15]. Meanwhile, FeO₄²⁻ possesses strong oxidizing properties that degrade the extracellular polymer substances (EPSs) of sludge. Specifically, iron ions neutralize the charge and help compress the EPS double layer, thus improving the dewatering effect [16].

A majority of sludge treatment studies involving K₂FeO₄ have primarily assessed its effect on sludge dewatering [17]. Further, it was reported that the addition of a new green conditioning agent K₂FeO₄ does not produce any toxic or harmful substances in the sludge [15]. Despite the conditioning agent facilitating the dewatering process, the remaining substances may affect the pyrolysis products of the sludge, especially the production of polluting gas [18]. Few studies have been conducted concerning the effect of sludge dewatering conditioners on sludge pyrolysis. Sludge conditioner mainly contain sludge disintegration conditioner [19], flocculating conditioner [7], and sludge skeleton builder [5]. Previous studies have focused on the effects of flocculating conditioner and sludge skeleton builder on sludge pyrolysis and the release of contaminant elements [5,20], while the effects of sludge disintegration conditioner on sludge pyrolysis have not been reported. Potassium ferrate (K₂FeO₄) is widely used in sludge conditioning and dewatering. In addition to flocculation, it has a certain disintegrating effect on sludge [15,16], organic matter such as proteins and sugars in sludge will be released into the liquid phase, and with the separation of dewatering and sludge dry matter, the properties of sludge before and after disintegration will be greatly changed, which may affect sludge pyrolysis.

Therefore, the effect of K₂FeO₄ as a sludge conditioning conditioner on sludge pyrolysis characteristics and the contaminant element release characteristics were investigated in this study. We investigated the effect of K₂FeO₄ on the evolution and release characteristics of N-, S-, and Cl-containing contaminants during sludge pyrolysis. Furthermore, we analyzed the dried sludge after K₂FeO₄ conditioning using the thermogravimetric mass spectrometry (TG-MS) and pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS) methods. Several solid-state reaction models were used to perform a kinetic analysis of the pyrolysis process.

2. Materials and Methods

2.1. Sample Collection and Preparation

The experimental samples in this study were collected from the municipal surplus sludge of the Xintian Sewage Treatment Plant, Wanzhou, Chongqing, China. After sample collection, samples were conditioned with K₂FeO₄ for 24 h. The conditioning ratio was 177.32 g of K₂FeO₄ per kg of dry sludge. The conditioned sludge was dewatered with vacuum filtration to obtain a K₂FeO₄ sludge cake, and a raw sludge cake was obtained by dewatering the untreated sludge. The two kinds of cake were dried at 105 ± 5 °C in constant-temperature drying drum bellows to obtain the dry sludge samples, which were ground to particles of less than 0.15 mm in size. Finally, the samples were placed in sealed bags for storage until subsequent experiments could be conducted.

2.2. Preparation of Sludge Biochar

The sludge samples were placed in a closed tubular electric furnace, and 100 mL/min of nitrogen (N₂) was injected into the furnace for 10 min before heating. Then, the tubular electric furnace was heated from room temperature to 800 °C for 30 min at a heating rate of 15 °C/min. Immediately after the completion of pyrolysis, the samples were cooled to room temperature in a dryer for subsequent experiments.

2.3. Physical and Chemical Properties

The sludge samples were analyzed according to the industrial analysis method for solid biomass fuel (GB/T 28731-2012). A Vario EL cube CHNS elemental analyzer (Elementar, Langenselbold, Germany) was used to determine the C, H, N, and S contents of the samples. The chlorine content in the samples was determined using the determination method for chlorine in solid biomass fuel (GB/T 30729-2014).

2.4. TG-MS and Py-GC/MS Analysis

TG-MS (Thermo plus EV2/Thermo Mass Photo, Japan) was used to determine the weight loss curves and N/S/Cl contaminant release characteristics of the sludge samples. Approximately 10 mg of sludge sample was placed in a platinum crucible in a thermogravimetric (TG) furnace and then heated from room temperature to 800 °C at a rate of 15 °C/min under N₂-release conditions of 300 mL/min.

Py-GC/MS experiments were carried out using a thermal cracker (EGA/PY-3030D, Japan) combined with GC/MS (CMS-QP2010 SE, Shimadzu, Japan). The pyrolyzer was preheated to 800 °C, and the sample was put inside and pyrolyzed for 24 s in a helium atmosphere. The volatiles from the pyrolysis were collected and separated with gas chromatography (7890B, agilent, American) and identified with mass spectrometry (MS). The column temperature program consisted of room temperature to 50 °C over 1 min, 50 °C to 80 °C at 5 °C/min, and finally 80 °C to 300 °C at 15 °C/min. Subsequently, the temperature was maintained at 300 °C for 20 min. The chromatographic peaks were identified according to the NIST database, and the compounds that appeared in more than 70% of samples were classified. The compounds obtained were divided into six classes: aliphatic hydrocarbons, oxygen-containing compounds, nitrogen-containing compounds, chlorine compounds, and sulfur compounds. Compounds in which the number of carbon atoms less than or equal to 7 were classified into group C7≤, while compounds in which the number of carbon atoms greater than 7 were classified into group C7>.

2.5. Dynamic Analysis

The specific method of dynamical analysis is shown in File S1.

3. Results and Discussion

3.1. Effect of Potassium Ferrate on the Physicochemical Properties of Sludge

Table 1. Proximate and ultimate analyses of the sludge samples.

Sample	Proximate Analysis (wt %)				Ultimate Analysis (wt %)
	Mad	Aad	Vad	FCad	C	H	O 1	N	S	Cl	Fe
RSS 2	1.88	31.00	60.63	6.49	34.47	5.25	21.48	6.68	0.913	0.21	2.23
PF–SS	1.04	39.45	55.24	4.27	30.46	4.70	18.71	5.46	0.94	0.28	9.59

¹ O (wt %) = 100 − (C + H + N + S + Cl + A_ad). ² The data are from our previously published paper [20].

shows the industrial and elementary analyses of the sludge samples. The results of the industrial analysis showed that compared with the original sludge, the sludge conditioned by ferric chloride (FC-SS), and ferric sulfate (FS-SS), respectively [20], the percentage of moisture, volatile content, and fixed carbon in the potassium-permalate-conditioned sludge (PF-SS) decreased, while the percentage of ash increased. This could be attributed to potassium permalate being an inorganic substance that has a cracking effect on the sludge, causing the organic substances, e.g., protein and polysaccharide, in the sludge to be released from the solid phase into the liquid phase and discharged with the extracted water during the dewatering process. Wu et al. have proposed potassium ferrate to be a strong oxidant with a highly effective sludge-disintegration ability [14]. In addition, the results of elemental analysis showed that compared with that in RSS, FC-SS, and FS-SS, the percentage of C, H, O, and N in PF-SS decreased, while the percentage of Fe increased significantly. This may be because the reduction of Fe⁶⁺ in potassium ferrate to Fe³⁺, which was retained in the negatively charged sludge, reduced the organic components in the dry sludge. In addition, the release of protein and polysaccharide may reduce the content of C, H, O, and N in PF-SS.

3.2. Effect of Potassium Ferrate on the Sludge Thermal Decomposition Characteristics

Figure 1 shows the thermogravimetric (TG) and reciprocal thermogravimetric (DTG) curves of the PF-SS and RSS treatments. The pyrolysis process consisted of three stages. The first stage (room temperature–180 °C) was the drying stage, with RSS and PF-SS weight decreasing by 3.16% and 4%, respectively. The changes in the sludge at this stage were small and mainly originated from the loss of free water and light volatile compounds (e.g., aliphatic and aromatic structures) [21]. The second stage (180–550 °C) was the main stage of sludge pyrolysis mass loss, with a 48.92% and 44.69% weight reduction in RSS and PF-SS, respectively. In this stage, the organic matter in the sludge is gradually carbonized and converted into fixed carbon and noncondensable gases (CO, CO₂, etc.), while organic compounds and tar were released [22]. According to the DTG curve, temperatures of 180–380 °C (range Ⅰ) may be related to the decomposition of biodegradable substances. Additionally, there was a shoulder peak at temperatures of 380–550 °C (range Ⅱ), which may be due to the presence of overlapping organic matter in ranges I and II of the pyrolysis process and may involve the decomposition of organic substances (e.g., polysaccharides, proteins, lipids, and acids) [23].The peak DTG and total mass loss at this stage was less for PF-SS than for RSS due to the higher organic content in RSS than for PF-SS and the increased iron content in the sludge, which affected the percentage of organic matter content. Further, the DTG of PF-SS peaked at 300 °C, which was lower than that of RSS which peaked at 320 °C. These differences may be due to the catalytic effect of the Fe introduced by the conditioning, which reduced the temperature required for the thermal decomposition of some components of organic matter [24]. The third stage (above 550 °C) was mainly the release of small amounts of thermal decomposition gases, which may be associated with the decomposition of residual carbon and organic and inorganic salts (carbonate, sulfate, and organic salts, including carboxylate) [25]. According to the DTG curve, the PF-SS treatment appeared to increase the decomposition of inorganic salts during the pyrolysis stage. In line with the results of our previous report [20], the thermal decomposition characteristics of PF-SS were similar to those of FC-SS and FS-SS.

3.3. Kinetics Analysis

The kinetic parameters of RSS and PF-SS for two temperature ranges (Range Ⅰ: 180–380 °C and Range Ⅱ: 380–550 °C) of the second stage pyrolysis reaction are listed in Table S1, including the activation energy, finger front factor, and linear regression coefficient (R²). In Range Ⅰ, the activation energy of PF-SS was smaller than that of RSS, and the R² of the first order (F1) in the chemical reaction order had a better R² of 0.999. In this stage, the activation energy required to initiate the reaction process for PF-SS was lower because of the high organic matter content of the RSS Itself and the high iron content of the sludge conditioned with potassium permalate. Additionally, the cracking effect of potassium permalate also reduced the organic matter content in the sludge; thus, PF-SS consequently had a lower organic matter content. Meanwhile, in Range Ⅱ, the activation energy of PF-SS was slightly higher than that of RSS, possibly because more chemical energy was required for the subsequent reaction with potassium permalate and because other organic matter components that are difficult to decompose were present. Furthermore, chemical reaction order 2 (F2) was 0.999 for Range II of RSS and PF-SS. F1 and F2 are therefore suitable for the pyrolysis of the sludge samples under study. In summary, PF-SS required less activation energy during pyrolysis, indicating that the addition of iron-containing conditioning agents reduced the activation energy required for sludge pyrolysis, making it easier for the sludge to be pyrolyzed to produce volatiles.

3.4. Pyrolytic Gas Evolution

3.4.1. Release of C/H/O-Containing Substances

Figure 2 shows the release pattern of the main C/H/O gas products (CH₄, H₂O, CO, C₂H₆, C₃H₄, and CO₂) released at different temperatures during pyrolysis of the sludge samples, and

Table 2. MS peak areas of the main C/H/O-containing permanent gases during the pyrolysis of the sludge samples.

Compound	Molecule	m/z	RSS 1	PF-SS
Methane	CH4	16	9.09 × 10−7	1.14 × 10−6
Water	H2O	18	1.02 × 10−5	1.08 × 10−5
Carbon monoxide	CO	28	2.30 × 10−6	2.71 × 10−6
Ethane	C2H6	30	1.83 × 10−7	1.70 × 10−7
Propyne	C3H4	40	1.36 × 10−7	1.05 × 10−7
Carbon dioxide	CO2	44	4.00 × 10−6	4.84 × 10−6

¹ The data are from our previously published paper [20].

presents the peak integral areas of the C/H/O gas products. As shown in Figure 2a, methane (CH₄) shows a clear main peak over the temperature interval of 170–400 °C, which is generated by the release of large-molecule compounds via cleavage into small-molecule compounds, probably due to the promotion of residual iron ions in the sludge [20]. A shoulder peak at the temperature interval of 400–590 °C probably resulted from the cleavage of methoxy clusters [26]. Additionally, the CH₄ peak of PF-SS appeared earlier than that of RSS, proving that PF-SS could generate CH₄ at a lower pyrolysis temperature. Figure 2b shows the H₂O release curve with a peak at a temperature of 100 °C, where the production of H₂O gas was mainly caused by the evaporation of adsorbed water on the sample surface. Additionally, significant H₂O peaks were also observed when the pyrolysis temperatures were at 285 °C and 485 °C, which may be related to the bonding of -OH, -O, and H₂ generated at high temperatures and the decomposition of oxygenated compounds [27]. Notably, the generation of CO mainly depends on the C-O-C or C=O double bond breakage [28]. Figure 2c shows the release curve of the gas CO, which is produced mainly by C-O-C or C=O double bond breakage. When the temperature was lower than 200 ℃, CO release tended to decrease, which may be related to N2 being adsorbed on the sample surface. The curve peaks after 200 °C and tends to increase after 550 °C; the PF-SS clearly tends to rise more than the RSS, and the production of CO at this stage is related to the Boudouard reaction (C + CO₂ ↔ 2CO) [29]. The reaction of Fe₂O₃ (generated at high temperatures with K₂FeO₄) and CO leads to a shift in the Boudouard reaction towards CO generation, allowing the C present in the sludge to participate in the reaction. Additionally, Fe₂O₃ can promote C-C bond breakage and increase CO emission, which also confirms that the PF-SS weight loss rate is greater than that of RSS at temperatures above 700 °C (Figure 1) [30]. Figure 2f shows the CO₂ release curve, and it is clear from the figure that there is a peak over the temperature interval of 170–550 °C and that the peak of PF-SS is higher than that of RSS. According to the results in Table 2, the peak areas of CH₄, H₂O, CO, and CO₂ generated during PF-SS pyrolysis were larger than those of RSS, which proves that the iron contained in the sludge promotes the generation of permanent gas products containing C/H/O during the pyrolysis process. The results in Figure 2d,e and Table 2 show that C₂H₆ and C₃H₄ yields from PF-SS pyrolysis are smaller than those from RSS, which may be due to the low volatile organic matter content and high ash content of PF-SS compared to RSS. According to the results of a previous study [20], both ferric chloride and ferric sulfate promote the production of CH₄, CO, C₂H₆, C₃H₄, and CO₂. In this study, K₂FeO₄ promoted the production of CH₄, CO, and CO₂, and the CH₄ and CO produced by PF-SS were lower than those of FC-SS, which may be due to the low content of C, H, and O in PF-SS (as shown in Table 1).

3.4.2. N-Containing Pollutants

Figure 3 shows the release patterns of the main N-containing pollutant gas products (NH₃, HCN, C₂H₃N, HNCO, C₂H₇N, NO₂, C₃HN, C₃H₃N, C₄H₅N, and C₅H₅N) released during the pyrolysis of different sludge samples at different temperatures.

Table 3. Peak integrated areas of gas products containing N pollutants.

Compound	Molecule	m/z	RSS 1	PF-SS
Ammonia	NH3	17	2.72 × 10−6	2.49 × 10−6
Hydrogen cyanide	HCN	27	9.04 × 10−7	6.98 × 10−7
Acetonitrile	C2H3N	41	5.23 × 10−7	4.52 × 10−7
Isocyanic acid	HNCO	43	5.49 × 10−7	4.75 × 10−7
Ethylamine	C2H7N	45	1.38 × 10−7	8.78 × 10−8
Nitrogen dioxide	NO2	46	4.93 × 10−8	4.27 × 10−8
Cyanoacetylene	C3HN	51	6.55 × 10−8	4.58 × 10−8
Acrylonitrile	C3H3N	53	7.37 × 10−8	4.59 × 10−8
Pyrrole	C4H5N	67	4.47 × 10−8	3.99 × 10−8
Pyridine	C5H5N	79	2.29 × 10−8	2.44 × 10−8

¹ The data are from our previously published paper [20].

lists the peak integrated areas of the gaseous products containing N pollutants. The results show that in sludge pyrolysis, the main N-containing gaseous products that are converted to N species in sludge are HCN, NH₃, C₂H₃N, and HNCO. Further, the amount of N-containing contaminants released from the PF-SS treatment was less than that released from the RSS treatment, except for C₅H₅N. HCN release showed a wide temperature range, with two peaks in PF-SS and three peaks in RSS, indicating the existence of different HCN formation pathways [31]. The peaks in the range 300–550 °C can be explained by the formation of functional -CN groups attached to hydrogen radicals as a result of the dehydrogenation cleavage of some N-amines and the cleavage of the nitrogen heterocycle contained in some residue samples. At temperatures greater than 550 °C, the release of HCN may be due to the conversion of some amines to nitrile, which generates a large amount of HCN and subsequently lowers NH₃ production [32]. The NH₃ gas release also showed a relatively wide temperature range, and both the PF-SS and RSS showed three peaks. For PF-SS, except for the first peak, the other two peaks were lower in height than that of RSS. The first peak below 200 °C represents mainly the conversion of inorganic ammonium salts and unstable proteins. The second peak between 200 °C and 400 °C represents mainly the hydrogenation of amino-N compounds from the thermal decomposition of proteins to form NH₃. Lastly, the third peak was around 500 °C, which may be the reason for the formation of NH₃ through tar N cleavage in combination with hydrogen radicals [33]. The release curve of acetonitrile (C₂H₃N) was similar to that of HCN, showing a weak shoulder peak near 360 °C. They may have similar formation pathways, such as dehydrogenation, and undergo azaheterocyclic cleavage. In addition, the -CN functional group had halogen-like properties, which can lead to the bonding with hydrocarbon groups and alkane substitution reactions. HNCO was mainly produced in the 200–520 °C interval, with two distinct spikes at 270 °C and 480 °C demonstrating two different formation pathways. In conclusion, PF-SS released fewer nitrogenous contaminants than did RSS during pyrolysis, which may be partly due to the iron compounds in PF-SS immobilizing N into the char as more stable nitrogen species (pyridine-N and pyrrole-N) [34]. However, the cracking effect of potassium permeate on the sludge leads to the N-containing organic matter, such as proteins, being released from the sludge into the liquid phase and discharged during the conditioning phase [20]. Some studies have shown that after the sludge is conditioned with potassium pertechnetate, extracellular polymeric substances (EPS) and cellular material are released into the aqueous phase, consisting primarily of polysaccharides and proteins [35].Compared to the results of our previous studies on the effect of iron-containing conditioner on sludge pyrolysis gas [20], the amount of all the N-containing pollutant gas products produced by PF-SS was lower than that produced by FC-SS and FS-SS. This may be due to the low content of the N element in PF-SS (as shown in Table 1).

3.4.3. S-Containing Pollutants

Figure 4 shows the release pattern of the primary S-containing pollutant gas products (H₂S, CH₃SH, COS, SO₂, CS₂, C₂H₆OS, and C₂H₆O₂S) as a function of temperature, obtained from different sludge samples.

Table 4. Peak integrated areas of products of gases containing S pollutants.

Compound	Molecule	m/z	RSS 1	PF-SS
Hydrogen sulfide	H2S	34	5.74 × 10−8	1.61 × 10−8
Methyl mercaptan	CH3SH	48	7.14 × 10−8	2.57 × 10−8
Carbonyl sulfide	COS	60	3.91 × 10−8	1.14 × 10−8
Sulfur dioxide	SO2	64	7.70 × 10−8	2.16 × 10−8
Carbon disulfide	CS2	76	7.89 × 10−9	9.35 × 10−9
Dimethyl sulfoxide	C2H6OS	78	1.56 × 10−8	2.59 × 10−8
Dimethyl sulfone	C2H6O2S	94	1.98 × 10−8	1.53 × 10−8

¹ The data are from our previously published paper [20].

lists the peak integral areas of the S gas-containing products. The results showed that the PF-SS treatment significantly reduced the release of H₂S, CH₃SH, COS, and SO₂ compared with the RSS treatment. Pyrolysis caused release of large amounts of hydrogen-containing radicals, such as CH₃ and H, which combine with S-containing radicals and are released as CH₃SH and H₂S, respectively [36]. Figure 4a represents the release curve of H₂S. Two peaks were evident from the graph: the first spike occurred at 300 °C, and the main source of release at this stage was the cleavage of aliphatic-S; the second spike was observed at 480 °C, and the release at this stage was mainly attributed to the cleavage of aromatic-S. Further, a micropeak was also present at 700 °C, which was likely related to the thermal decomposition of stable thiophene-S species in tar and charcoal [37]. In addition, a slight shoulder peak in H₂S occurred at temperatures exceeding 600 °C, which may be related to the thermal decomposition of stable thiophene-S species in tar and char [38]. Figure 4b shows that the release of CH₃SH is similar to that of H₂S, with two main peaks occurring at 340 °C and 450 °C, respectively. The main source is the decomposition of some organic sulfides including thiols, polysulfides, disulfides, dialkyl sulfides, and alkyl-aryl sulfides in the sample. The release of COS may be related to the combination of C=O radicals with S radicals as well as the reaction of CO₂ and H₂S under high-temperature conditions as represented in the following equation: ${CO}_2+H_{2}S\to CO+H_{2}O$. However, as the temperature increased, a large amount of H₂ was produced, and H₂ and COS reacted ($H_2+COS\to CO+H_{2}S$), which led to a reduction in COS content. The increase in iron promotes the activation of hydrogen atoms, and this also explains the apparently lower release of gas from PF-SS compared to RSS, as shown in Figure 4c [39]. Figure 4d shows that SO₂ production is mainly due to oxidation by oxygen-containing reactive radicals, such as $SH+·OH\to {SO}_2+H_2$ [40]. In summary, the sludge conditioned with potassium permalate released significantly fewer S-containing contaminants than did the original sludge, and this was likely because of the inhibited formation of sulfate by elemental iron which also promoted the activation of hydrogen atoms, thereby facilitating the hydrogenation saturation of sulfur compounds and the cleavage of aromatic rings. Compared to the results of our previous studies on the effect of iron-containing conditioner on sludge pyrolysis gas [20], the amount of all the S-containing pollutant gas products produced by PF-SS was lower than that produced by FC-SS and FS-SS. This may be due to the low content of the S element in PF-SS (as shown in Table 1). In addition, the iron (elements) content in PF-SS was significantly higher than that in RSS, FC-SS, and FS-SS. Iron ions have a catalytic oxidation effect on H₂S and SO₂, converting H₂S to S and SO₂ to SO₄²⁻ and thus further reducing the content of H₂S and SO₂ produced by PF-SS [41].

3.4.4. Cl-Containing Pollutants

Figure 5 shows the release patterns of the main Cl-containing pollutant gas products (HCl, Cl₂, and chlorinated organic pollutants) released during the pyrolysis of different sludge samples at different temperatures.

Table 5. Peak integrated areas of gaseous products containing Cl contaminants.

Compound	Molecule	m/z	RSS 1	PF-SS
Hydrogen chloride	HCl	37	4.32 × 10−8	2.79 × 10−8
Methyl chloride	CH3Cl	50	5.49 × 10−8	4.26 × 10−8
Vinyl chloride	C2H3Cl	62	1.02 × 10−8	8.49 × 10−9
Nitrosyl chloride	NOCl	65	3.10 × 10−8	3.02 × 10−8
Chlorine	Cl2	71	1.63 × 10−8	2.05 × 10−8
Allyl chloride	C3H5Cl	77	2.74 × 10−8	2.23 × 10−8
Chlorobenzene	C6H5Cl	113	6.18 × 10−9	1.48 × 10−9

¹ The data are from our previously published paper [20].

lists the peak integrated areas of the Cl-containing gaseous products. The results indicated that except for Cl₂, the release of Cl-containing contaminants from PF-SS was lower than that from RSS. RSS and PF-SS produce chlorinated compounds mainly through the cleavage of chlorine-containing organics, as shown in Equations (S4)–(S6). The possible source of HCl was its formation from certain chlorides present in the sludge during pyrolysis. This is shown in Equation (S7), where M represents the different metal elements present in the sludge in the form of metal ions. In addition, the reaction of inorganic chlorides in the sludge with SiO₂ could also produce HCl and Cl₂ as shown in Equations (S8) and (S9). Cl₂ reacts with the NO, produced during pyrolysis, to produce NOCl, as shown in Equation (S10). Compared to the results of the previous study [20], the amount of all of the Cl-containing pollutant gas products produced by PF-SS was lower than that produced by FC-SS but higher than that produced by FS-SS. This may be due to the contents of Cl element being as follows: FS-SS < PF-SS < FC-SS (as shown in Table 1).

Equations (S4)–(S10) are shown in the File S2.

3.5. Pyrolytic Volatiles Products

Figure 6 lists the classification and percentage peak areas for the pyrolysis chromatogram identification of PF-SS versus RSS. Noticeably, the area of the aliphatic hydrocarbon peak from the PF-SS pyrolysis (26.73%) was greater than that of RSS pyrolysis (21.87%). The peak area of the benzene derivative of PF-SS (26.32%) was lower than that of RSS (26.11%). The generation of benzene and derivatives is primarily caused by the decomposition of lignin and partly by the aromatization of cellulose [42]. Additionally, the catalytic reformation and decomposition of benzene derivatives may be due to the iron element of PF-SS [43]. The PF-SS has a lower peak area for oxygen compounds than does RSS. In addition, the peak area of the C7> group released by PF-SS pyrolysis was lower than that of the RSS pyrolysis, while the peak area of the C7≤ group was higher than that of RSS. Compared to the results of the previous study [20], the peak area of the C7≤ group produced by PF-SS was larger than that of FC-SS, while the peak area of the C7> group produced by PF-SS was smaller than that of FC-SS. This may be because iron ions have a catalytic effect, and the content of iron in PF-SS is higher than that in FC-SS. Thus, the iron ions catalyzed the cleavage of the C7> group into the C7≤ group [44]. The identified fraction group of compounds produced during the pyrolysis of PF-SS was higher. These observations demonstrate that Fe promotes the cleavage of macromolecular compounds and the conversion to light tar and syngas, reducing the complexity of the compounds produced by pyrolysis [45]. Based on the percentage peak areas of chlorinated and S-containing compounds in the sludge, potassium permalate conditioning likely reduces the number of chlorinated compounds in the tar, while having no significant effect on the formation of S-containing compounds in the tar phase. Reduction in chlorinated compounds in the tar likely led to the increase in the release of Cl₂.

4. Conclusions

In this study, sludge was conditioned with K₂FeO₄, dewatered, and dried. The effects of K₂FeO₄ as a sludge-dewatering conditioning agent on the pyrolysis characteristics of sludge and the release characteristics of N-, S-, and Cl-containing contaminants during pyrolysis were investigated using TG-MS and Py-GC/MS. The following conclusions were obtained:

(1)

The pyrolysis of sludge samples was divided into three stages (room temperature–180 °C, 180–550 °C, and above 550 °C). The largest weight loss occurred in the 180–550 °C interval, which primarily involved the pyrolysis of organic matter and fixed carbon, with less residual mass in the PF-SS than the RSS conditions. The dewatering pretreatment with K₂FeO₄ had a facilitatory effect on sludge pyrolysis.

(2)

Sludge conditioned with K₂FeO₄ requires less energy for the pyrolysis process. This step also reduces the activation energy, and the type and complexity of volatiles in sludge pyrolysis, making the sludge more susceptible to the reaction.

(3)

Compared with RSS, PF-SS promoted the release of C/H/O gases but inhibited that of N/S/Cl-contaminated gases.

(4)

PF-SS increased the calorific value of tar and improved the chemical stability of bio-oil. The addition of iron promotes the cracking of macromolecular compounds into light tar and syngas as well as the formation of aliphatic hydrocarbons and the catalytic reformation and decomposition of benzene. Moreover, the PF-SS treatment moderately reduces the formation of compounds containing nitrogen, sulfur, and chlorine. Compared to the results of the previous study, the amount of CH₄ and CO as well as that of the N-containing and S-containing pollutant gas products produced by PF-SS was lower than that produced by FC-SS, which may be due to the low content of C, H, O, N, and S in PF-SS. The amount of all the Cl-containing pollutant gas products produced by PF-SS was lower than that produced by FC-SS, but higher than that of FS-SS.

In this study, the effect of potassium ferrate as a sludge-disintegration conditioner on sludge pyrolysis was analyzed in detail, with particular focus given to the release of polluting elements, including N/S/Cl, to provide a theoretical basis for the environmental pollution problems that may arise during the sludge pyrolysis after potassium ferrate conditioning. However, potassium ferrate is only one of the common sludge-disintegration conditioners, and the effects of other common sludge-disintegration conditioners such as potassium permanganate and persulfate on sludge pyrolysis were not investigated in this study; therefore, further comparative analysis of the effects of sludge-disintegration conditioners on sludge pyrolysis is needed in subsequent studies, which may provide a firmer theoretical basis for the selection of conditioners in the sludge conditioning–dewatering–pyrolysis system process.