Synergistic Effects on the Co-pyrolysis of Agricultural Wastes and Sewage Sludge at Various Ratios.

This study investigated the co-pyrolysis of blends of sewage sludge (SS) with rice husk (RH) and with hemp straw (HS) at different ratios by using thermogravimetry (TG) and its rate (DTG, derivative TG) analysis at heating rates of 10, 20, and 30 K/min. The resulting kinetic parameters of activation energy (E a) were calculated by both Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose models, followed by comparison of experimental values with calculated values to reveal the synergistic effects of SS/RH and SS/HS. With increasing additions of RH or HS to SS, a gradual decreasing trend in the experimental pyrolysis temperature range was evident, ranging from 144.5 to 95.2 °C for SS/RH and from 144.5 to 88.8 °C for SS/RH. Moreover, such temperature ranges were 6.7-20.4 °C less than the calculated values at the same blending ratio. The fitting results of the two kinetic models showed that with the same SS mass ratio, the experimental E a * (average activation energy) of both SS/RH and SS/HS were less than the calculated E a *. Especially, the experimental E a * of 7SS-3RH was lower around 43.8% than the calculated E a *, whereas the experimental E a * of 3SS-7HS was lower by about 39.4% than the calculated E a *. Synergistic analysis demonstrated that the co-pyrolysis of RH or HS with SS at various mass ratios presented obvious synergistic effects and then the decrease of E a. The mechanism experiment showed that the co-pyrolysis of SS/HS may promote the decrease of E a by changing the co-pyrolysis gas products or by increasing the overflow of volatile matter and then forming intermediate transition products, while SS/RH may accelerate the decrease of the E a by using an appropriate K addition ratio from RH as a metal catalyst.

Introduction

Sewage sludge (SS) originating from wastewater treatment plants may be considered a nuisance, which is a key source for secondary environmental contamination, considering the presence of organic and inorganic pollutants.[1] Meanwhile, the SS production quantity is rapidly increased this decade, which sharply emerges in China, reaching about 30 million tons of wet sludge with 80% moisture content annually.[2] Therefore, the energy and resource utilization disposal methods of SS have attracted great attention.[3,4] At present, pyrolysis is one of the main treatment methods of SS due to less air pollution and carbon effects as compared with incineration.[5] Besides, its products mainly include biochar, bio-oil, and bio-gas, and these products may be further refined to chemicals with higher economic values.[6] However, the main limitation of the high-efficiency pyrolysis of individual SS is its relatively high moisture and ash content, resulting in higher energy consumption.[4] For this reason, improving the energy utilization of SS in pyrolysis has become an urgent research issue.

Recently, in order to increase the energy utilization value of SS, there have been many studies on the co-pyrolysis of SS and biowaste. Previous studies found that there were both synergistic and antisynergistic effects of co-pyrolysis of SS and biowaste. For instance, Wang et al.[6] reported that the co-pyrolysis of SS and wheat straw reduced the heat demand of the pyrolysis process and showed a strong synergistic effect at appropriate amounts of wheat straw addition. Ding and Jiang[7] showed that SS decomposition, without external heat supply, by means of co-pyrolysis of SS with waste biomass was practically achieved. However, Guo et al.[8] reported that the co-pyrolysis of SS and corn stalk had an antisynergistic effect on NOx emission at low heating temperatures, and CO2 was the main product during co-pyrolysis. Ruiz-Gómez et al.[9] presented that there was no significant synergistic effect during the thermal decomposition of mixed SS and manure by thermogravimetry (TG) analysis. In turn, these results revealed that both synergistic and antisynergistic interactions occurred during the co-pyrolysis of SS and biowaste, which led to variations in the reaction activation energy (Ea) and pyrolysis characteristic. These variations mainly depended on the intrinsic characteristics of biowaste as well as the blending ratio. For instance, the two types of biowaste rice husk (RH) and hemp straw (HS) are obviously different in their intrinsic nature for renewable energy generation. On one hand, as one of the widespread agricultural wastes in China,[10,11] RH contains low volatile matter and high lignin content. On the other hand, as a versatile crop waste in China that produces the most industrial hemp in the world,[12] HS contains high volatile matter and low ash content. Overall, the pyrolysis characteristic and Ea were different during the monopyrolysis of individual RH and HS. Therefore, the co-pyrolysis properties might not be simply speculated by the behaviors of pure biowaste as expected.[13]

It can be seen that the detailed thermal behavior during co-pyrolysis, especially the synergistic mechanism, has not been revealed in depth, despite the fact that it is important for the improvement of energy utilization efficiency of SS and biowaste. The main objective of this work is to disclose the synergistic and antisynergistic mechanisms of the co-pyrolysis of SS with two types of biowaste. Moreover, the specific objectives include (1) investigating the intrinsic properties of pure biomass such as SS, RH, and HS and the individual relationship to the monopyrolysis behavior; (2) analyzing the kinetic change of Ea during the co-pyrolysis of the corresponding blends by means of Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS) methods; and (3) elucidating the synergistic and antisynergistic mechanisms of the co-pyrolysis of SS with two types of biowaste.

Results and Discussion

Physical Characteristics of Pure Biomass

The chemical composition, ultimate analysis, and elemental analysis of SS, RH, and HS are shown in Table . The chemical composition results demonstrate that RH had 11.78% cellulose, 31.60% hemicellulose, and 18.96% lignin contents, whereas HS contained 16.79% cellulose, 32.15% hemicellulose, and 5.21% lignin contents. These values demonstrate that RH had a higher lignin content than HS. The ultimate analysis results show that the oxygen (O) content in SS, RH, and HS was 66.7, 59.1, and 47.6%, respectively, whereas the carbon (C) content in SS, RH, and HS was 25.3, 35.0, and 44.9%, respectively, and the contents of H, N, and S in SS, RH, and HS were all less than 5%. As reported in Table , the results of mineral element analysis indicate that the iron (Fe) content in SS was 108 times that of RH and 692 times that of HS, whereas the potassium (K) content in RH was 35.2% higher than that in SS and 326.6% higher than that in HS. Furthermore, SS contained a large amount of elements such as phosphorus (P), silicon (Si), and magnesium (Mg), while these elements were poor in RH and HS.[14] The results of XRF analysis show that SS ash had the highest content of Fe2O3 at 16.9% among all samples, and the main component in RH ash was SiO2. In addition, the K2O content of RH ash was 156.0% times that of HS ash and was 278.6% times that of SS ash. This result is similar to the mineral elemental analysis of raw materials.

Table 1

Characteristics of Pure SS, RH, and HSa

sample	SS	RH	HS	sample	SS ash	RH ash	HS ash
cellulose content (%)	-	11.78	16.79	SiO2 (%)	22.5	49.3	3.4
hemicellulose content (%)	-	31.60	32.15
lignin content (%)	-	18.96	5.21
C (%)	25.3	35.0	44.9	Fe2O3 (%)	16.9	0.7	0.3
H (%)	4.2	4.9	6.4
Ob(%)	66.7	59.1	47.6
N (%)	3.3	0.7	0.6	Al2O3 (%)	9.5	0.1	1.2
S (%)	0.5	0.3	0.5
Fe (mg/g)	34.60	0.32	0.05
P (mg/g)	11.24	0.13	0.06	K2O (%)	1.4	3.9	2.5
K (mg/g)	2.84	3.84	0.90
Si (mg/g)	2.25	0.20	0.34
Mg (mg/g)	2.51	0.27	0.25	P2O5 (%)	4.7	0.7	1.6

The hyphen represents no experimental data.

Oxygen content was determined by the difference.

FTIR and XRD Analyses of Pure Biomass

Figure a shows the Fourier transform infrared (FTIR) spectra for SS, RH, and HS, respectively. For SS, RH, and HS, the stretching vibrations of hydrogen-bonded OH groups (3600–3200 cm–1) were clearly observed, resulting from the raw material with a higher content of moisture. Furthermore, the peaks at 2935 and 2885 cm–1 attributed to the aliphatic C–H stretching bands were evident in SS, RH, and HS.[15] The C=O stretching vibrations in the range of 1740–1700 cm–1 for the carboxyl, aldehyde, ketone, and ester groups were identified in the spectra of RH and HS but were absent in SS.[15] The band at 1595 cm–1 in SS, RH, and HS was ascribed to C=C and C=N.[16] The broad band in 1100–950 cm–1 was attributed to the overlapping peaks of P–O and C–O, which was evident in SS, RH, and HS. However, due to the low P content, the main peaks of RH and HS in this region could be inclined to the vibration of C-O. These results indicate that the abundant O-containing functional groups were detected on the surface of SS, RH and HS. Figure b shows the XRD patterns of the SS, RH, and HS. Briefly, quartz (SiO2) at 21.6° was identified in the SS, RH, and HS. The phosphate mineral whitlockite [(Ca,Mg)3(PO4)2] at 28.1° was only identified in SS, resulting from the higher P content in SS. This XRD result is consistent with the elemental analysis of pure RH, HS, and SS.

Figure 1

FTIR (a) and XRD (b) patterns of SS, RH, and HS.

Physical Characteristics of Blends

As presented in Table , for pure SS, RH, and HS, the experimental proximate analysis indicate that SS contained a higher ash content at 46.00% than RH at 23.93% and HS at 2.67%. This is because SS contained a large amount of metal, which is consistent with the above elemental analysis results. Besides, HS contained higher volatile matter (VM) content at 81.12% than SS at 45.05% and RH at 62.23%, respectively, while the fixed carbon (FC) of HS is similar to that of SS and RH. Among the blends of SS/RH and SS/HS, SS/HS had higher VM and FC content and lower ash content than SS/RH of the same ratio. These experimental results were all similar to the corresponding calculated values, which indicate that there was no synergistic effect on the proximate analysis of blends of SS and biowaste.

Table 2

Proximate and HHV Analyses of SS/RH and SS/HSa

	proximate analysis (wt %, db)
sample	ASHexp	ASHcal	VMexp	VMcal	FCexp	FCcal	HHVexp (MJ/kg)	HHVcal (MJ/kg)
SS	46.00	46.00	45.05	45.05	8.95	8.95	8.79	8.79
7SS–3RH	39.59	39.38	50.23	50.20	10.18	10.41	9.43	9.59
5SS–5RH	34.80	34.97	54.00	53.64	11.20	11.39	10.18	10.13
3SS–7RH	30.80	30.55	56.86	57.08	12.34	12.37	10.84	10.67
RH	23.93	23.93	62.23	62.23	13.83	13.83	11.48	11.48
SS	46.00	46.00	45.05	45.05	8.95	8.95	8.79	8.79
7SS–3HS	33.39	33.00	55.78	55.87	10.83	11.13	10.58	10.35
5SS–5HS	24.05	24.34	63.11	63.09	12.84	12.58	11.30	11.39
3SS–7HS	15.90	15.67	69.61	70.30	14.49	14.03	11.99	12.43
HS	2.67	2.67	81.12	81.12	16.21	16.21	13.99	13.99

Note: db represents dried base.

Interestingly, with the increasing addition of RH or HS, the experimental high heating value (HHVexp) of SS/RH or SS/HS showed a steadily increasing trend, respectively, which was parallel to the resulting calculated high heating value (HHVcal). Therefore, there was no synergistic effect on HHV of SS/RH or SS/HS. This may be due to the combustion reaction between the mixture and oxygen; the reaction was completed instantly and CO2 was released. Therefore, it does not show a synergistic effect, and it only presents its own mixture composition’s intristic characteristics.

TG and Derivative TG Analyses of Blends

Figure a,b represents the TG and derivative TG (DTG) curves of SS, RH, HS, and their resulting blends. Overall, as Figure a was compared with Figure b, it can be found that the blends of SS/RH contained lower weight loss and weight loss rate than SS/HS at the same mass ratio.

Figure 2

TG and DTG curves of blends at different mass ratios for (a) SS/RH and (b) SS/HS.

The TG curves in Figure a,b can be separated into four stages based on the degradation of different components. In the first stage, loss of water was observed onto SS, RH, HS, and the blends at the temperature range from 25 to 150 °C. The second stage extends to 400 °C, during which the whole hemicellulose and part of cellulose from RH and HS and organic materials from SS degrade simultaneously.[17] This result is considered as the first main decomposition stage or active pyrolysis zone because of the larger percentage of mass loss occurring in this stage.[18] The third stage ranges from 400 to 600 °C, during which the cellulose residues from SS, RH, HS, and the blends as well as the nonbiodegradable organic material thermally decompose.[14,17] This thermal decomposition of cellulose in this stage is gradual because it is likely to be destroyed at a higher pyrolysis temperature due to its macromolecular structure.[19] The fourth stage ranges from 600 to 900 °C, during which lignin from RH and HS continued to degrade. However, the inorganic material from SS decomposed above 600 °C due to the thermal decomposition of fixed carbon and inorganic contents.

The DTG curves in Figure a,b show that all the blends of SS/RH or SS/HS had slight temperature peaks at the first stage due to the loss of water from SS, RH, and HS and had obvious temperature peaks at the second stage resulting from the degradation of hemicellulose, cellulose, and organic materials. Additionally, with the increasing addition of RH and HS in the blends, the temperature peak intensity at the second stage showed a steadily increasing trend. Furthermore, at the third and fourth stages, the rate of weight loss was relatively flat due to the degradation of small amount of the remaining nonbiodegradable organic material.

The pyrolysis characteristic parameters for the blends of SS/RH or SS/HS are shown in Table . The pyrolysis characteristic parameters include the initial pyrolysis temperature (Ti), final pyrolysis temperature (Tf), and pyrolysis temperature range. It can be found that the increasing addition of RH or HS to SS resulted in the continuous increase of experimental Ti ranging from 250.5 °C for SS to 271.6 °C for RH and 278.0 °C for HS, respectively. Similarly, the increasing addition of RH or HS into SS resulted in the steady fall of experimental Tf ranging from 395 °C for SS to 366.8 °C for RH and 366.8 °C for HS, respectively. This could be attributed to the nature of Ti and Tf in pure RH and HS. Therefore, with the increasing addition of RH or HS to SS, the experimental pyrolysis temperature range presented a gradual drop from 144.5 to 95.2 °C for the blend of SS/RH and from 144.5 to 88.8 °C for the blend of SS/HS. This shortened pyrolysis temperature range was related to the rising initial and the falling final pyrolysis temperature for the related blends, which could be attributed to the increasing portion of volatile matter in the blends, and this result was consistent with previous studies.[20,21]

Table 3

TG and DTG Data for Blends at the Heating Rate of β = 10 °C/min

	experimental result			calculated result
blend samples	initial pyrolysis temperature Ti (°C)	final pyrolysis temperature Tf (°C)	experimental pyrolysis temperature range (°C)	initial pyrolysis temperature Ti (°C)	final pyrolysis temperature Tf (°C)	calculated pyrolysis temperature range (°C)
SS	250.5	395	144.5	250.5	395.0	144.5
7SS–3RH	262.6	385.6	123.0	256.8	386.5	129.7
5SS–5RH	268.0	377.9	109.9	261.0	380.9	119.9
3SS–7RH	269.0	364.4	95.4	265.3	375.2	109.9
RH	271.6	366.8	95.2	271.6	366.8	95.2
SS	250.5	395	144.5	250.5	395.0	144.5
7SS–3HS	270.2	377.6	107.4	258.7	386.5	127.8
5SS–5HS	270.7	371.1	100.4	264.2	380.9	116.7
3SS–7HS	276.1	368.3	92.2	269.7	375.2	105.5
HS	278.0	366.8	88.8	278.0	366.8	88.8

Furthermore, in the blending of either SS/RH or SS/HS, the experimental pyrolysis temperature ranges show a similar trend to their corresponding calculated results, but the former was lower by 6.7–20.4 °C than the latter at the same blending ratio and blending precursors. These results were due to the synergistic effect during co-pyrolysis of the blending of SS/RH or SS/HS. The shortening of the experimental pyrolysis temperature range, especially the falling of the experimental Tf, is beneficial to the pyrolysis of the sample at a lower temperature, leading to lower energy demand for the pyrolysis reaction and higher energy utilization.[22]

Kinetic Analysis of Blends

To calculate the Ea values using the FWO and KAS methods, the linear regression results of the relationship between ln β and 1/T as well as ln(β/T2) and 1/T at six conversion rates (α = 0.1–0.6) were obtained, as shown in Figure S1. Besides, α ranging from 0.1 to 0.6 was chosen to study the Ea value because the considerable pyrolyzable fraction might occur at this range.[23] According to the overall linear regression results, at the heating rates of 10, 20, and 30 °C/min, all the correlation coefficients (R2) were larger than or equal to 0.95, either using the FWO or KAS model. This result reflects that both methods can simultaneously explain the co-pyrolysis process in this study.

The variation of Ea for the co-pyrolysis process versus α is shown in Figure a,b. Either for the blends of SS/RH shown in Figure a or for the blends of SS/HS shown in Figure b, almost parallel Ea values were calculated by both the FWO and KAS models, considering the similar linear regression curves for both methods. Also, by comparison with Figure a,b, at the same α ranging from 0.1 to 0.6, all blends had the similar changing trend for the Ea values. Especially, with the increase of α from 0.1 to 0.4, Ea of blends significantly increased. This result reflects that at the beginning of conversion (α = 0.1), the low Ea value may be due to the decomposition of molecules with weak bonds or volatile components in SS and the two types of biowaste, whereas with the increasing conversion (α = 0.2–0.4), the high Ea value was attributed to the decomposition of strong bond molecules including all of the hemicellulose and part of the cellulose.[24] Then, as α continued to increase from 0.4 to 0.6, Ea was relatively stable at 195.9–214.4 kJ/mol for the blend of SS/RH while at 173.9–223.6 kJ/mol for the blend of SS/HS in FWO model, which was attributed to the thermal decomposition of cellulose with an ordered structure of the linear organic compound.[25,26] This result is consistent with the mass loss percentage in the TG analysis results. Thus, according to the changing trend of Ea at various α, it was found that the pyrolysis reaction mechanism of the blends of SS/RH or SS/HS may be the unification of several complex pyrolytic reaction mechanisms.[27]

Figure 3

Activation energy (Ea) values at different α values of the blends in FWO and KAS models: (a) RH addition and (b) HS addition.

The synergistic effect analysis of the comparison of the experimental Ea* (average activation energy) with the calculated Ea* is shown in Figure a,b. Besides, the solid lines and the dotted lines are referred to the experimental Ea* and calculated Ea*, respectively. Interestingly, at the same SS mass ratio ranging from 0.3 to 0.7, the experimental Ea* values were all less than the calculated Ea* values. Then, the decreased Ea* during co-pyrolysis in the blends of either SS/RH or SS/HS may lead to a shortened pyrolysis range compared with that of the unblended material, consistent with the above experimental pyrolysis temperature ranges, and then an improved pyrolysis rate. Thus, a synergistic effect between SS and biowaste has been established. Afterward, the synergistic effect on Ea* became remarkable as the mass ratio of SS/RH was 7/3 (the experimental Ea* was lower (about 43.8%) than the calculated Ea*), whereas that of SS/HS was 3/7 (the experimental Ea* was lower (about 39.4%) than the calculated Ea*). On the one hand, considering the blends of SS/RH, it was obvious that RH had a higher K content as compared with SS; meanwhile, for the blends of SS/HS, HS has a higher VM content than SS, and then K content and VM content maybe two contributing factors, which led to the following confirmation experiment to illustrate the synergistic effect mechanism.[28] In addition, the decrease of Ea* may reduce the critical energy required for the pyrolysis reaction, increase the activated molecular fraction, enhance the effective collision times, and consequently improve the pyrolysis reaction rate.[29,42]

Figure 4

Comparison of the experimental Ea* (average activation energy) with the calculated Ea* in SS/RH blends and SS/HS blends using (a) FWO and (b) KAS models.

Mechanism Analysis

Co-pyrolytic Volatile Products

Gas yield and product distribution during the co-pyrolysis of SS/HS blends (xSS–yHS) with different mixing ratios are shown in Figure . In the main pyrolysis stage interval (250–400 °C), the pyrolytic gas products of SS consist of 86% of CO2, 7% of H2, and 7% of CH4, while those of HS consist of 68% of CO2, 25% of CO, 2% of CH4, and 1% of H2. The gas products from the co-pyrolysis of SS and HS were mainly a large amount of CO2 and CO and a small amount of H2. Among these gas products, various organic compounds of SS and HS may be fully pyrolyzed to generate CO2 in large quantities, while CO may mainly come from a large amount of cellulose and hemicellulose that have not been completely cracked in the pyrolysis process of HS.[30] Besides, a small amount of H2 and CH4 may mainly come from a small amount of macromolecular organic matter that is difficult to degrade in the SS, which may be due to its slow pyrolysis reaction rate.

Figure 5

Gas yield and product distribution during the co-pyrolysis of SS/HS blends with different mixing ratios.

Interestingly, when the addition ratio of HS was 0.3, due to the synergistic effect during the co-pyrolysis of SS and HS, the gas product transformed from the mixture of CO2, CO, H2, and CH4 derived from the single pyrolysis of SS and HS to only CO derived from co-pyrolysis. This is because the formation enthalpy of CO is lower than that of CO2. Therefore, the pyrolysis of 7SS–3HS easily generated much more CO, which may be consistent with the reduction of pyrolysis Ea shown in the above kinetic analysis.

When the proportion of HS was increased to 0.5 or 0.7, the gas products were mainly CO2 and H2. The composition of gas products was similar to that of SS during single pyrolysis, but the pyrolysis Ea of the blends was much lower than that of SS. These results indicate that, due to the synergistic effect, the reaction pathways of CO2, CO, H2, and CH4 produced by co-pyrolysis may be significantly different from that of single SS pyrolysis, which will promote the reduction of Ea of co-pyrolysis.[31] This synergistic effect may be attributed to the increase in the content of volatile compounds during pyrolysis with the increase of HS addition. This may be beneficial to the generation of intermediate transition products in the refractory organic compounds of SS, resulting in the fall of the Ea of complexes.[32] These intermediate products may be further pyrolyzed to produce CO2, CO, H2, and CH4.[33]

Effect of Alkaline-Earth Metals on Co-pyrolysis Ea

The variation of experimental Ea* (average activation energy) versus K addition ratio based on the K content in RH for the blends of SS and potassium carbonate (SS/K) during the co-pyrolysis process is shown in Figure . There was similarity in the experimental Ea* values for both FWO and KAS models, considering the similar linear regression curves for both methods. Also, by comparison of the FWO and KAS methods, with K addition ratio ranging from 0 to 0.7, all SS/K blends had the similar changing trend for the Ea* values.

Figure 6

The experimental Ea* (average activation energy) in SS/K blends using both FWO and KAS models.

With the increase of K content, the experimental Ea* of SS/K blends decreased first and then increased, and all the experimental Ea* of SS/K blends were less than that of pure SS. This result was consistent with the change in the trend of the co-pyrolysis experimental Ea* of SS and RH with different additions, indicating that K-containing compounds may be used as positive catalysts to reduce the Ea* of SS pyrolysis. However, with the increasing addition of K content, the experimental Ea* showed a slight upward trend, which may be due to the excessive reaction of the K compound catalyst. The excessive reaction may lead to a continual reaction between the unreleased gas products and the reactants, resulting in other side reactions with higher Ea.[34]

Conclusions

The synergistic effect has been investigated by adding RH or HS into SS during co-pyrolysis. TG/DTG data demonstrated that for the blends of either SS/RH or SS/HS, experimental pyrolysis temperature ranges were shorter than the calculated values. For the blends of either SS/RH or SS/HS, the synergistic analysis indicated that all the experimental Ea* values were lower than their calculated Ea* values at the same mass ratio. Especially, as the mass ratio of SS/RH was 7/3, the experimental Ea* of the blend was lower (about 43.8%) than the calculated Ea*, whereas as the mass ratio of SS/HS was 3/7, the experimental Ea* of the blend was lower (about 39.4%) than the calculated Ea*. Finally, synergistic analysis demonstrated that co-pyrolysis of RH or HS with SS at various mass ratios obviously presented the synergistic effects and then reduced the Ea.

The mechanism analysis showed that as the HS addition ratio was 0.3, the pyrolysis experimental Ea* reduced because the synergistic effect changed the pyrolysis products and generated a large number of CO with a lower formation enthalpy. Moreover, as the HS addition ratio was 0.5 or 0.7, the pyrolysis experimental Ea* continued to reduce because the synergistic effect generated intermediate transition products during pyrolysis and changed the reaction pathway of co-pyrolysis of HS and SS. In addition, the K compound in RH ash may be used as a positive catalyst to reduce the pyrolysis Ea during the co-pyrolysis of RH and SS, but the capability to reduce the Ea decreased with the increase of the RH addition ratio, which may be due to the emergence of other side reactions with a high Ea energy caused by the excessive K compound spontaneously.

Materials and Methods

Preparation of Sewage Sludge and Biowaste

Sample diversity was achieved by selecting two types of biowaste, namely RH and HS, which were both collected in Guangzhou, China. Additionally, SS was collected from Guangzhou, China. Prior to experiments, all samples were dried at 105 °C for 24 h and sieved through 100 mesh (0.15 mm) screens for the following experiments.

Proximate analysis was performed by a muffle furnace using the ASTM D-5142 method. More information related to the operation conditions could be found in our previous study.[35] The contents of cellulose, hemicellulose, and lignin in biowaste were determined by the NREL method. The detailed methods were described in the study of Sluiter et al.[36] Ultimate analysis (CHNS) was performed in an automated Vario EL cube elemental analyzer (Elementar, Langenselbold, Germany), while the oxygen content was determined by differences. The total metal content in soil samples was determined by inductively coupled plasma–optical emission spectrometry (iCAP 6000; Thermo Fisher) following microwave digestion (Milestone ETHOS A, Italy) in the three-acid mixture (concentrated HF–HNO3–HClO4). To get a precise insight on the metals in the char-derived ash samples, the latter was analyzed by XRF spectrometry (PANnalytical, Axios Advanced) to measure the major metal oxides. The experimental higher heating value (HHVexp) was determined using a calorimetric bomb (YX-ZR, China). FTIR spectra were determined using a Bruker Vertex 70 spectrophotometer in the range of 400–4000 cm–1 with 32 scans for the samples. XRD (D8 ADVANCE, Germany) was carried out with a Cu Kα radioactive source (k = 0.154 nm) at 36 kV/30 mA with a scan speed of 4 °C/min.

Experimental Design and Procedure

The blends were mixed with various mass ratios of SS to RH or HS to carry out the co-pyrolysis of SS with biowaste. The resulting blends were denoted as xSS–yRH or xSS–yHS, and x and y were represented as the blending mass ratio. For example, 3SS–7HS represented a mixture of SS and HS at the mass ratio of 3/7. A specific diagram of the experimental design is shown in Figure .

Figure 7

Flow diagram of the experimental design and procedures.

Thermogravimetric Analysis

Thermal degradation behaviors of SS, RH, HS, and their resulting blends were determined by a TG analyzer (NETZSCH STA449C Jupiter) under a nitrogen atmosphere at 30 mL/min. A fine mixture powder of 8–12 mg was placed in a small alumina crucible and heated from room temperature to 900 °C at different heating rates of 10, 20, and 30 °C/min.

Kinetic Theory

The pyrolysis process of biomass is represented by general reactions,[37,38] which are shown in the Supporting Information. The integral form,[16,17]G(α), for the isoconversional model can be expressed based on the conversion rate and reaction kinetics and rearranging yields (eq )[39]where G(α) = integral conversion and x = E/(RT). However, P(x) is an integral form of temperature and has no exact solution; and there are only a few numerical approximations.

Then, in this study, the integral isoconversional methods of FWO and KAS were applied to calculate the kinetic parameters of co-pyrolysis of blends with different blending ratios.[37]

FWO Method

The FWO method employs Doyle’s approximation as the model-free method to calculate the Ea of a material.[40] It can be expressed as follows

The Ea was calculated by plotting a graph between ln β and 1/T.

KAS Method

The KAS method is an isoconversional method and is used to calculate the kinetic energy of a material.[40] It can be expressed as follows

The Ea was calculated by plotting a graph between ln(β/T2) and 1/T.

Synergistic Effect

An experimental effect that results between two substances or materials, which is greater than the sum of their individual effects is known as a synergistic effect.[41] On the contrary, it indicates an antisynergistic effect.[42] This experimental effect may be due to the interaction of molecules of both substances, especially carbon and hydrogen.[42,43] Besides, the calculated values represented the summation of the behaviors of SS and biowaste (RH or HS) in the blends. In this study, when the experimental value is higher or less than the calculated value, it indicates a synergistic or antisynergistic effect, respectively. When the experimental value is equal to the calculated value, no synergistic effect exists.[43] The calculated value can be expressed as followswhere x is the mass ratio of SS in the blend, and xWSS and (1 – x) WBIO are the corresponding indictor values of SS and biowaste including proximate analysis, HHV, Ti, Tf, and Ea. Ea* is obtained from the mean of total Ea values corresponding to conversion (α).

Confirmation Experiment

In order to investigate the effect of the alkaline-earth metal in RH on the SS pyrolysis Ea, various ratios of K2CO3 were added to SS for further TG experiments, and the resulting blends were denoted xSS–yK. Besides, continuous pyrolysis was carried out to elucidate the relationship between the volatile matter escape from HS and the Ea change. The blends were put into the quartz tube at room temperature and pyrolyzed at the heating rate of 10 °C/min and a N2 flow rate of 20 mL/min. The evolved gas was collected at the main pyrolysis stage (250–400 °C), and the collected gas was detected by gas chromatography (Agilent 19095P-Q04, GC).