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. 2020 Feb 14;5(7):3389–3396. doi: 10.1021/acsomega.9b03608

Catalytic Gasification of Sewage Sludge in Supercritical Water: Influence of K2CO3 and H2O2 on Hydrogen Production and Phosphorus Yield

Weijin Gong 1,*, Zizheng Zhou 1, Yue Liu 1, Qingyu Wang 1, Lina Guo 1
PMCID: PMC7045557  PMID: 32118153

Abstract

graphic file with name ao9b03608_0003.jpg

In this work, the catalytic gasification of sewage sludge in supercritical water was investigated in a batch reactor (460 °C, 27 MPa, 6 min), and the separate and combined effects of the catalyst on the H2 production and phosphorus yield were investigated. The experimental results indicated that K2CO3 alone improved the H2 yield, gasification efficiency (GE), and carbon gasification efficiency (CE). The largest H2 yield of 54.28 mol/kg was achieved, which was approximately three times that without a catalyst. Furthermore, the inorganic phosphorus (IP) yield increased with the addition of K2CO3. However, when H2O2 was added, the H2 yield quickly decreased with increasing H2O2 coefficient, and more than 97.8% of organic phosphorus (OP) was converted into IP. The H2 yield increased with the addition of various K2CO3/H2O2 ratios, whereas the IP yield decreased.

1. Introduction

Sewage sludge is an inevitable product of wastewater treatment. The amount of sewage sludge has increased with the construction and expansion of wastewater treatment plants while stricter policies have limited its disposal.1 Sewage sludge contains complex organic materials mainly consisting of carbohydrates (∼14%), proteins (∼40%), lipids (∼10%), lignin (∼17%), and ash (30–50%).2

Various techniques for converting sewage sludge into useful secondary energy have been developed.3−5 One of the methods for energy recovery is the gasification of sewage sludge in supercritical water.6 Supercritical water gasification (SCWG) is regarded as an emerging economical and environmentally friendly technology for sewage sludge treatment and hydrogen production.7

In recent years, many research studies on the supercritical water gasification of sewage sludge have been reported.8−11 To improve the hydrogen yield, a small amount of oxidant was added in the SCWG process. Both hydrogen yield5,12 and removal efficiency of organic matter5 in sewage sludge could be improved with the addition of oxidant (H2O2) in the SCWG of sludge.

The gasification reaction could be accelerated with the addition of suitable catalysts in SCWG, resulting in an increase in hydrogen production. These catalysts include metals, carbon, metal oxides, and alkalies (e.g., KOH, NaOH, K2CO3, and Na2CO3). Alkali catalysts are widely employed for high hydrogen production and inhibition of tar/char formation.13−15

When the K2CO3 catalyst was added in the SCWG process, the gasification efficiency (GE) was enhanced.7 Gong et al.8 also reported that the hydrogen yield was higher when a combined catalyst with different Ni/NaOH ratios was used. A multicomponent catalyst consisting activated carbon and Na2CO3 has been reported to enhance production and the removal of organics.5

Phosphorus is essential for the growth of life. As phosphate resources have gradually become scarce, it is necessary to look for a sort of recyclable, environmentally friendly phosphorus resource.16 Sewage sludge has a high phosphorus content, approximately 8% (w/w), which makes it a potential source of the P nutrient.17 Phosphorus recovery from sewage sludge using different methods (e.g., chemical oxidation, subcritical water process, and SCWG) has been investigated.18−23 Also, the gasification of sewage sludge in supercritical water for hydrogen production and phosphorus recovery has been investigated.11,24,25 However, systematic studies on the effects of K2CO3 and H2O2 on hydrogen production and phosphorus recovery during SCWG of sludge have not been reported.

In this work, the catalytic gasification of sewage sludge in supercritical water with the addition of K2CO3 and H2O2 was studied, and the effects of the catalyst separately and the combined effects of K2CO3 and H2O2 on hydrogen production and phosphorus recovery were investigated.

2. Results and Discussion

2.1. Effects of K2CO3 and H2O2 Individually on Gas and Phosphorus Yield

Figure 1 illustrates the effects of varying the amount of the K2CO3 additive on the H2 gas and phosphorus yields from the SCWG of sludge at 460 °C and 27 MPa after 6 min. As shown in Figure 1a, an increase in H2 yield was obtained as the K2CO3 concentration increased. When the K2CO3 concentration increased from 0 to 8 wt %, the H2 yield increased from 19.86 to 54.28 mol/kg, which was approximately three times that without catalyst. Unlike the H2, the CO2 yield increased from 7.13 to 12.57 mol/kg and then reduced to 10.65 mol/kg as the K2CO3 increased. An inflection point at a concentration of 6 wt % for K2CO3 was observed. Meanwhile, the CO and CH4 yields hardly changed with an increase in the K2CO3 concentration. These results agreed with previous reports, which showed slight changes in the CO and CH4 yields with increasing K2CO3.26,2726,27 The comparison of this study with previous literature28−35 is shown in Table 1.

Figure 1.

Figure 1

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Effect of the amount of K2CO3 on the gas and phosphorus yields from the SCWG of sludge at 460 °C and 27 MPa after 6 min: (a) gas yield, (b) GE and carbon gasification efficiency (CE), (C) inorganic phosphorus (IP) and organic phosphorus (OP) yields, and (d) total phosphorus (TP) yield in the liquid sample and solid residue.

Table 1. Comparison of Hydrogen Yield Obtained from This Study with the Literature (460 °C, 27 MPa, 6 min).

feed catalyst hydrogen yield refs
sludge KOH, K2CO3, NaOH, Na2CO3 15.49 mol/kg (28)
dewatered sewage sludge H2O2/Ni 0.79–1.28 mol/kg of organic matter (25)
sewage sludge none 38.5–39.4 vol % (24)
dewatered sewage sludge NaOH, KOH, K2CO3, Na2CO3, Ca(OH)2 3.45 mol/kg of organic matter (15)
dewatered sewage sludge formic acid 10.07 mol/kg of organic matter (29)
dewatered sewage sludge NaOH and Ni 4.8 mol/kg of matter (30)
horse manure Na2CO3 5.31 mmol/g (31)
semicoke K2CO3 85.90 mol/kg (32)
timothy grass KOH 30.6 mol/kg (33)
bagasse KOH 75.6 mol/kg (34)
landfill leachate NaOH 70.05 mol/kg (35)
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As shown in Figure 1b, similar to the change in the CO2 yield, the GE and CE values increased from 49.78 and 257.34% to 80.8 and 368.09%, respectively, with an increase in the amount of the K2CO3 additive. As the K2CO3 concentration continued to increase, the GE and CE reduced to 73.2 and 326.78%, respectively.

As mentioned above, K2CO3 had an important catalytic effect on the H2 yield, and the catalytic effect was enhanced as the K2CO3 concentration increased. Several previous studies36−38 have reported that alkali catalysts could significantly enhance the water gas shift reaction (eq 1)

2.1. 1

Regarding the catalytic mechanism, the primary role of the K2CO3 catalyst is to improve the water gas shift reaction. The catalytic effect of the K2CO3 catalyst can be explained by the following reactions.

K2CO3 reacts with water to produce KHCO3 and alkali.

2.1. 2

Alkali reacts with CO to form formate salt.

2.1. 3

Formate salt reacts with water to form H2.

2.1. 4

KHCO3 decomposes to form CO2.

2.1. 5

The phosphorus yield in a liquid sample was also determined, as shown in Figure 1c,d. The OP yield and the amount of TP in the liquid production decreased as the amount of K2CO3 increased, whereas the IP yield and the concentration of TP in the solid residue increased. The OP yield decreased from 14.6 to 5.08% when the K2CO3 concentration increased from 0 to 8 wt %. OP was mostly converted into IP within a short time. However, the amount of TP in the residue increased with the amount of K2CO3, which increased from 36.97 to 52.54%. This result is in agreement with a previous work.39

Sullivan and Tester40 reported that OP could be completely eliminated and converted into phosphate in the final residue within a short time. Under supercritical operating conditions, gasification proceeds quickly and converts compounds containing phosphorus to orthophosphoric acid, which can be neutralized to its corresponding salts and precipitated in the solid residue. The OP contained in sewage sludge is composed of a collection of phospholipids, phosphonic compounds, and nucleic acids, which could be eliminated and converted to IP completely by increasing the K2CO3 concentration, resulting in an increase of IP and TP in the solid residue.

The gas production and phosphorus yield were measured when different amounts of H2O2 (reflected by different oxidation coefficients varying from 0.5 to 2.5) were added to the SCWG process of sludge at 460 °C and 27 MPa after 6 min, as shown in Figure 2.

Figure 2.

Figure 2

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Effect of the amount of H2O2 on the gas and phosphorus yields from the SCWG of sludge at 460 °C and 27 MPa after 6 min: (a) gas yield, (b) GE and CE, (C) IP and OP yields, and (d) TP yield in the liquid production and solid residue.

As shown in Figure 2a, the H2 yield quickly decreased from 3.37 to 0.14 mol/kg with an increase in the H2O2 coefficient from 0.5 to 2.5, whereas a H2 yield of 19.86 mol/kg was obtained without the addition of the oxidant. A similar decrease in the CH4 content from 8.16 to 0.15 mol/kg was also observed. However, the CO2 yield increased directly from 7.13 to 29.77 mol/kg as the H2O2 coefficient increased from 0 to 2.5.

As shown in Figure 2b, when the H2O2 coefficient was 1.5, the GE was higher than 100%. It increased from 114.86 to 131.6% as the H2O2 coefficient increased from 1.5 to 2.5. In addition, the CE also increased from 333.54 to 502.08% with the addition of H2O2. As shown in Figure 2c,d, when the H2O2 coefficient was 2.5, over 97.8% of OP was converted to IP. The amount of TP in liquid production decreased as the H2O2 coefficient increased from 0 to 2.5, whereas the concentration of TP in the solid residue increased. The total mass balance of phosphorus was between 97.7 and 98.8%.

It has been reported that a large amount of oxidant can enhance the oxidation reaction and convert combustible gases into H2O and CO2, thereby leading to a decreased H2 yield.28 Our experimental results coincided with these published results. The experimental results indicated that a greater number of carbohydrates and phosphorus organic compounds were decomposed with the addition of H2O2. As the H2O2 coefficient increased, the oxidation reaction became dominant in the supercritical water. H2 and CH4 were converted to CO2 and H2O, with an excess of H2O2, which led to a decrease in the H2 yield. The maximum H2 yield was obtained without the addition of H2O2. In addition, an increase in the decomposition of organic compounds and the combustion of gas containing carbon with the addition of an oxidant to the SCWG process contributed to a higher CO2 yield, as well as increased GE and CE values.

The organic phosphorus contained in sewage sludge is composed of a collection of phospholipids, phosphonic compounds, nucleic acids, phosphoproteins, etc. In SCWG, the sufficient H2O2 primarily promotes the oxidation and decomposition of organics in sludge into small molecule intermediate products at the initial and middle stages. The P–C bond is promoted to crack with the addition of H2O2, and OP was converted completely to IP, resulting in an increase of the IP yield and TP yield in the solid residue.

2.2. Effect of the Combination of K2CO3 and H2O2 on Gas and Phosphorus Yields

The effect on the gas and phosphorus yields by simultaneously adding K2CO3 and H2O2 was investigated at 460 °C and 27 MPa after 6 min.

We first assumed that the catalytic effects of H2O2 and K2CO3 were independent. When they were added in different proportions, the calculated value of hydrogen production for the mixture was assumed to be the sum of the results from H2O2 and K2CO3 catalyzation separately.

As shown in Table 2, there were large differences between the theoretical values and experimental data, indicating that H2O2 and K2CO3 have a synergistic effect on the reaction. The magnitude of the difference between the theoretical values and experimental data first increased and then decreased, which indicated that the K2CO3:H2O2 ratio also affected the reaction.

Table 2. Comparison of Calculated Values and Experimental Data of Hydrogen Yield from SCWG of Sludge (460 °C, 27 MPa, 6 min).

hydrogen yield (mol/kg)
with only H2O2 (wt %) with only K2CO3 (wt %) calculated value experimental data with K2CO3 and H2O2
19.86 19.86 39.72 5.47
3.37 21.50 24.87 8.75
2.29 37.81 40.10 22.65
0.59 47.52 48.11 23.70
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To investigate this further, both K2CO3 and H2O2 were added to the SCWG of sludge at various ratios. As mentioned above (shown in Figure 2), when the H2O2 oxidation coefficient was greater than 1.0, oxidation rather than gasification dominated in the supercritical water. To investigate the synergistic effect of K2CO3 and H2O2 on the SCWG reaction, the amount of H2O2 in the additive mixture (K2CO3 + H2O2) was below the theoretical demand for oxidation. When the K2CO3:H2O2 ratio was 1:1, the amount of K2CO3 was approximately 2 wt %. Five different K2CO3:H2O2 ratios were investigated. As shown in Figure 3a,b, the H2 yield increased from 5.47 to 29.06 mol/kg as the K2CO3:H2O2 ratio increased from 1:4 to 4:1. The H2 yield obtained with a K2CO3/H2O2 ratio of 1:1 was greater than 21.5 mol/kg, which was obtained with 2 wt % K2CO3. The CO2 yield decreased from 20.35 to 8.39 mol/kg and then increased to 11.4 mol/kg. The inflection point of the K2CO3/H2O2 ratio was observed at 1:1. The trends of the GE and CE were similar to that of the CO2 yield.

Figure 3.

Figure 3

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Effect of the ratio of K2CO3 to H2O2 on the gas and phosphorus yields from the SCWG of sludge at 460 °C and 27 MPa after 6 min: (a) gas yield, (b) GE and CE, (C) IP and OP yields, and (d) TP yield in the liquid production and solid residue.

The effect of the combination of K2CO3 and H2O2 on the phosphorus yield was also determined, as shown in Figure 3c,d. The IP yield decreased from 95.9 to 90.49% as the K2CO3/H2O2 ratio increased. This result coincided with those for K2CO3 and H2O2 separately. However, OP increased as the ratio increased. The fraction of TP in the solid residue first decreased from 71.94 to 46.78% and then increased to 50.23%. When the amount of H2O2 increased, OP was almost completely converted to IP.

As shown in Figure 3a, the mixing ratio of K2CO3 to H2O2 affected the H2 production and phosphorus yield. When the ratio of K2CO3 to H2O2 was less than 1:1, the amount of H2O2 was greater than that of K2CO3 and incomplete oxidation dominated in the SCWG process. Xu et al.27 reported that H2 and CO will be fully oxidized to H2O and CO2 in case of excess of oxidant, which results in a decreased H2 yield. However, because of the incomplete oxidation with insufficient H2O2, CO will be generated from hydrocarbons in sludge. The production of CO promoted the water gas shift reaction (eq 1), enhancing the H2 yield. With K2CO3, the yield of hydrogen is higher than that without K2CO3 (Table 3). The K2CO3 alkali catalyst had more effect on the water gas shift reaction. In contrast, the effect of K2CO3 on gasification was much stronger than that of H2O2 when the ratio was greater than 1:1. In the SCWG process, H2 was mainly derived from the reforming reactions of small intermediates.41

Table 3. Main Compounds in the Raw Sewage Sludge and Gasification Liquid Product in Supercritical Water (460 °C, 27 MPa, 6 min).

raw sewage sludge
 liquid gasification products
retention time (s) peak area (%) molecular formula retention time (s) peak area (%) molecular formula
2.55 13.25 C6H14O2 2.94 3.06 C6H12O
2.75 3.13 C41H58O 3.13 6.97 C5H5N
2.86 8.11 C4H10O3S2 5.74 3.19 C6H7N
3.86 2.88 C6H12O3 6.82 4.17 C6H8O
4.00 2.19 C6H12O3 8.45 2.32 C7H10O
5.63 3.75 C6H7ClN2 8.83 2.88 C8H12O
8.05 4.28 C10H30O5Si5 9.18 2.88 C8H12O
8.79 2.19 C4H8O3 9.29 10.98 C8H8O2
11.21 5.14 C14H22O 9.59 2.87 C8H10O
11.63 2.13 C20H36OSi 9.82 8.33 C8H10O
11.9 7.63 C20H27NO 10.67 9.56 C8H7N
11.99 3.27 C20H26O3 16.49 2.86 C12H43NO
12.07 7.46 C20H40O2      
12.15 5.18 C11H15Cl2O3PS2      
12.49 4.51 C17H34O2      
16.17 2.06 C18H35NO      
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In the absence of K2CO3 or H2O2, it is difficult for high-molecular-weight organic compounds to break down into low-molecular-weight compounds. When H2O2 was added, the gasification products mainly consisted of low-molecular-weight linear compounds. Meanwhile, complex organic matters such as cyclic compounds were enhanced to open the ring, resulting in the generation of low-molecular-weight linear compounds. Further gasification of these intermediates will be promoted with the addition of the K2CO3 catalyst. The compounds in the raw sewage sludge solution and the liquid gasification products are shown in Table 3.

As shown in Table 3, the main compounds in the raw sewage sludge solution are dimethoxybutane and other complex carbohydrates, with small amounts of nitrogen and phosphorus compounds. In the chromatogram, the ratio of the peak area of dimethoxybutane is 13.25%, which is approximately twice the area of any other compounds before gasification. After gasification in supercritical water, the main compounds in the liquid products are phenolic compounds and a few nitrogen compounds with small molecular weights. Methylbenzaldehyde is the main compound in the liquid products, with a 10.98% peak area ratio in the chromatogram. Therefore, K2CO3 in the additive mixture can effectively enhance the gasification of the intermediates as soon as possible. With the increase of the mass fraction of K2CO3, the TP content in solid residues decreased and the IP yield increased gradually. This indicated that organic phosphorus can be completely converted under the catalysis of K2CO3.

3. Conclusions

Few studies have attempted to simultaneously recover phosphorus and generate syngas from sludge using SCWG. However, the influence of alkali catalysts on phosphorus recovery in the SCWG of sludge has not been reported. The effect of the combination of K2CO3 and H2O2 on the hydrogen production and phosphorus yield was investigated for the first time in this work. When K2CO3 was used as a single additive, GE and CE were improved with the increase of K2CO3. Hydrogen yield was approximately three times that without a catalyst. The OP in the sewage sludge could be eliminated and converted to IP by increasing the K2CO3 concentration. However, when H2O2 was added separately to the SCWG process, the H2 yield sharply declined and carbon dioxide was the dominant gaseous gasification product. Furthermore, more than 97.8% of the OP was converted to IP. H2O2 and K2CO3 have a synergistic effect on the gasification. With the increase of the ratio of K2CO3 to H2O2, the K2CO3 catalyst had more effect on the water gas shift reaction, resulting in improvement of the hydrogen yield. Meanwhile, under the catalysis of K2CO3, more organic phosphorus can be completely converted into inorganic phosphorus (IP), resulting in a decrease of the TP yield and an increase of the IP yield.

4. Materials and Methods

4.1. Materials

Sewage sludge was provided by the Shuangqiao Sewage Treatment Plant in Zhengzhou, China and was stored in a preservation box at less than 4 °C prior to use. The compositions of the sewage sludge are given in Table 4. The K2CO3 catalyst and oxidant H2O2 (30 wt %) were purchased from Tianli Chemical Reagent Co., Ltd. and Tianjin Fuchen Chemical Reagent Co., Ltd., respectively. All of these reagents were analytically pure.

Table 4. Properties of Dewatered Sewage Sludge Used in the Experimentsb.

        element analysis (wt %)a
water content (wt %) organic mattera (wt %) asha (wt %) TOC (mg/L) C H Oc N P
79.0 ± 0.5 65.1 ± 0.4 20.2 ± 0.3 880.0 ± 5.0 7.36 ± 0.3 15.50 ± 0.5 55.66 ± 0.5 1.27 ± 0.2 3.81 ± 0.3
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a

On a dry basis, organic matter = 100% – ash% – fixed carbon%.

b

Water content = 100% – dry matter content.

c

By difference O% = 100% – ash% – C% – H% – N% – S%.

4.2. Supercritical Water Gasification System

The SCWG of the sewage sludge was performed in an HC276 stainless steel batch reactor, which was purchased from the Huaan Supercritical Extraction Co., Ltd., Nantong, Jiangsu, China. The reactor was described in detail in an earlier study.42

In a typical experiment, 3.0 g of sewage sludge with deionized water (100 mL) was added in the reactor. Different amounts of K2CO3 catalysts were mixed with the sewage sludge. Then, this mixture was loaded into the reactor. The reactor was electrically heated at an average heating rate of 3 °C/min. Once the reaction temperature (460 °C) was reached, the reaction was maintained for 6 min. At the end of each experiment, heating was stopped and the reactor was rapidly cooled to room temperature with cooling water. After the reactor had cooled, gas and liquid samples from the reactor were collected and submitted for analysis. Methods for the collection and separation of solid and liquid products have been described previously in detail.42

4.3. Analytical Methods

An Elementar Vario-III was used to analyze the organic elemental of sewage sludge. Furthermore, Agilent 890A/5975C was used to analyze the gasification liquid product, which was equipped with a DB-5MS column (30 m × 0.25 mm id, 0.25 μm film thickness) using high-purity helium as the carrier gas. Mass spectrometry was performed in a selected ion-monitoring mode. The mass spectrometry (MS) transfer line temperature was 270 °C, and the ion source temperature was 220 °C. The column temperature was initially held at 60 °C for 5 min, increased to 270 °C at a rate of 30 °C/min, and held at this temperature for 5 min. The injection volume was 10 μL. The total flow rate of the carrier gas was 25.0 mL/min, and the split flow rate was 20.0 mL/min.

The TP content in the dewatered raw sewage sludge and gasification solid residue were analyzed using an Agilent inductively coupled plasma-optical emission spectrometry (ICP-OES). The amount of TP in the liquid sample was analyzed using Chinese national methods (Ammonium molybdate spectrophotometric method, GB 11893-89). In addition, the SMT methodology was used to quantify the IP in the liquid sample.

The gas samples were analyzed with a gas chromatograph (Nanjing, GC5890), which was equipped with a thermal conductivity detector (TDX-01) to determine the H2, CH4, CO2, and CO concentrations.

The TP and IP yields were determined as follows

4.3. 6
4.3. 7

The OP yield was obtained by subtracting the IP yield from the TP yield.

The oxidation coefficient was defined as8

4.3. 8

The gas yield (GH2,GCH4, GCO2, and GCO), GE, and CE have been described in detail in an earlier study.42

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Project No. U1404523), Scientific and Technological Achievements in Henan Province (Project No. 192102310486), and China National Textile And Apparel Council (Project No. 2018040).

The authors declare no competing financial interest.

References

  1. Qian L.; Wang S. Z.; Xu D. H.; Guo Y.; Tang X. Y.; Wang L. F. Treatment of municipal sewage sludge in supercritical water: A review. Water Res. 2016, 89, 118–131. 10.1016/j.watres.2015.11.047. [DOI] [PubMed] [Google Scholar]
  2. Goto M.; Nada T.; Kodama A.; Hirose T. Kinetic analysis for destruction of municipal sewage sludge and alcohol distillery wastewater by supercritical water oxidation. Ind. Eng. Chem. Res. 1999, 38, 1863–1865. 10.1021/ie980479s. [DOI] [Google Scholar]
  3. Silva F. M. S.; Mahler C. F.; Oliveira L. B.; Bassin J. P. Hydrogen and methane production in a two-stage anaerobic digestion system by co-digestion of food waste, sewage sludge and glycerol. Waste Manage. 2018, 76, 339–349. 10.1016/j.wasman.2018.02.039. [DOI] [PubMed] [Google Scholar]
  4. Tang S.; Zheng C. M.; Zhang Z. T. Effect of inherent minerals on sewage sludge pyrolysis: Product characteristics, kinetics and thermodynamics. Waste Manage. 2018, 80, 175–185. 10.1016/j.wasman.2018.09.012. [DOI] [PubMed] [Google Scholar]
  5. Xu D.; Lin G. K.; Ma Z. J.; Guo Y.; Farooq M. U.; Wang S. Z. Partial oxidative gasification of sewage sludge in supercritical water with multi-component catalyst. Chem. Eng. Res. Des. 2017, 124, 145–151. 10.1016/j.cherd.2017.06.014. [DOI] [Google Scholar]
  6. Xu Z. R.; Zhu W.; Htar S. W. Partial oxidative gasification of municipal sludge in sub- and supercritical water. Environ. Technol. 2012, 33, 1217–1223. 10.1080/09593330.2011.618933. [DOI] [PubMed] [Google Scholar]
  7. Gasafi E.; Reinecke M. Y.; Kruse A.; Schebek L. Economic analysis of sewage sludge gasifification in supercritical water for hydrogen production. Biomass Bioenergy 2008, 32, 1085–1096. 10.1016/j.biombioe.2008.02.021. [DOI] [Google Scholar]
  8. Gong M.; Zhu W.; Xu Z. R.; Zhang H. W.; Yang H. P. Influence of sludge properties on the direct gasification of dewatered sewage sludge in supercritical water. Renewable Energy 2014, 66, 605–611. 10.1016/j.renene.2014.01.006. [DOI] [Google Scholar]
  9. Wang C.; Zhu W.; Chen C.; Zhang H.; Lin N. X.; Su Y. Influence of reaction conditions on the catalytic activity of a nickel during the supercritical water gasification of dewatered sewage sludge. J. Supercrit. Fluids 2018, 140, 356–363. 10.1016/j.supflu.2018.07.018. [DOI] [Google Scholar]
  10. Xu Z. R.; Zhu W.; Li M. Influence of moisture content on the direct gasification of dewatered sludge via supercritical water. Int. J. Hydrogen Energy 2012, 37, 6527–6535. 10.1016/j.ijhydene.2012.01.086. [DOI] [Google Scholar]
  11. Acelas N. Y.; López D. P.; Brilman D. W. F. W.; Kersten S. R. A.; Kootstra A. M. J. Supercritical water gasification of sewage sludge: Gas production and phosphorus recovery. Bioresour. Technol. 2014, 174, 167–175. 10.1016/j.biortech.2014.10.003. [DOI] [PubMed] [Google Scholar]
  12. Lin G. K.; Xu D. H.; Ma Z. J.; Wang S. Z.; Guo Y. Effect of Oxidation Coefficient on Products of Sewage Sludge Treatment in Supercritical Water. Energy Procedia 2017, 107, 357–362. 10.1016/j.egypro.2016.12.176. [DOI] [Google Scholar]
  13. Watanabe M.; Inomata H.; Osada M.; Sato T.; Adschiri T.; Arai K. Catalytic effects of NaOH and ZrO2 for partial oxidative gasification of n-hexadecane and lignin in supercritical water. Fuel 2003, 82, 545–552. 10.1016/S0016-2361(02)00320-4. [DOI] [Google Scholar]
  14. Xu D. H.; Wang S. Z.; Guo Y.; Tang X. Y.; Gong Y. M.; Ma H. H. Catalyzed partial oxidative gasification of phenol in supercritical water. Ind. Eng. Chem. Res. 2011, 50, 4301–4307. 10.1021/ie1016255. [DOI] [Google Scholar]
  15. Xu Z. R.; Zhu W.; Gong M.; Zhang H. W. Direct gasification of dewatered sewage sludge in supercritical water. Part 1: effects of alkali salts. Int. J. Hydrogen Energy 2013, 38, 3963–3972. 10.1016/j.ijhydene.2013.01.164. [DOI] [Google Scholar]
  16. Zhang H.; Zhang X. M. Phase behavior and stabilization of phosphorus in sub- and supercritical water gasification of cyanobacteria. J. Supercrit. Fluids 2017, 130, 40–46. 10.1016/j.supflu.2017.07.030. [DOI] [Google Scholar]
  17. Biswas B. K.; Katsutoshi I.; Hiroyuki H.; Keisuke O.; Hidetaka K. Leaching of phosphorus from incinerated sewage sludge ash by means of acid extraction followed by adsorption on orange waste gel. J. Environ. Sci. 2009, 21, 1753–1760. 10.1016/S1001-0742(08)62484-5. [DOI] [PubMed] [Google Scholar]
  18. Munir M. T.; Li B.; Mardon I.; Young B. R.; Baroutian S. Integrating wet oxidation and struvite precipitation for sewage sludge treatment and phosphorus recovery. J. Cleaner Prod. 2019, 232, 1043–1052. 10.1016/j.jclepro.2019.06.007. [DOI] [Google Scholar]
  19. Arakane M.; Imai T.; Murakami S.; Takeuchi M.; Ukita M.; Sekine M.; Higuchi T. Resource recovery from excess sludge by subcritical water process with magnesium ammonium phosphate process. J. Water Environ. Technol. 2005, 3, 119–124. 10.2965/jwet.2005.119. [DOI] [PubMed] [Google Scholar]
  20. Shi Y.; Luo G.; Rao Y.; Chen H. H.; Zhang S. C. Hydrothermal conversion of dewatered sewage sludge: Focusing on the transformation mechanism and recovery of phosphorus. Chemosphere 2019, 228, 619–628. 10.1016/j.chemosphere.2019.04.109. [DOI] [PubMed] [Google Scholar]
  21. Semerci N.; Kunt B.; Calli B. Phosphorus recovery from sewage sludge ash with bioleaching and electrodialysis. Int. Biodeterior. Biodegrad. 2019, 144, 104739 10.1016/j.ibiod.2019.104739. [DOI] [Google Scholar]
  22. Zhao Y.; Ren Q. Q.; Na Y. J. Potential utilization of phosphorus in fly ash from industrial sewage sludge incineration with biomass. Fuel Process. Technol. 2019, 188, 16–21. 10.1016/j.fuproc.2019.02.005. [DOI] [Google Scholar]
  23. Shiba N. C.; Ntuli F. Extraction and precipitation of phosphorus from sewage sludge. Waste Manage. 2017, 60, 191–200. 10.1016/j.wasman.2016.07.031. [DOI] [PubMed] [Google Scholar]
  24. Amrullah A.; Matsumura Y. Supercritical water gasification of sewage sludge in continuous reactor. Bioresour. Technol. 2018, 249, 276–283. 10.1016/j.biortech.2017.10.002. [DOI] [PubMed] [Google Scholar]
  25. Wang C.; Zhu W.; Gong M.; Su Y.; Fan Y. J. Influence of H2O2 and Ni catalysts on hydrogen production and PAHs inhibition from the supercritical water gasification of dewatered sewage sludge. J. Supercrit. Fluids 2017, 130, 183–188. 10.1016/j.supflu.2017.08.009. [DOI] [Google Scholar]
  26. Ishida Y.; Kumabe K.; Hata K.; Tanifuji K.; Hasegawa T.; Kitagawa K.; et al. Selective hydrogen generation from real biomass through hydrothermal reaction at relatively low temperatures. Biomass Bioenergy 2009, 33, 8–13. 10.1016/j.biombioe.2008.04.004. [DOI] [Google Scholar]
  27. Xu D.; Wang S. Z.; Tang X. Y.; Gong Y. M.; Guo Y.; Zhang J.; Wang Y. Z.; Ma H. H.; Zhou L. Influence of oxidation coefficient on product properties in sewage sludge treatment by supercritical water. Int. J. Hydrogen Energy 2013, 38, 1850–1858. 10.1016/j.ijhydene.2012.11.092. [DOI] [Google Scholar]
  28. Chen Y.; Guo L. J.; Cao W.; Jin H.; Guo S. M.; Zhang X. M. Hydrogen production by sewage sludge gasification in supercritical water with a fluidized bed reactor. Int. J. Hydrogen Energy 2013, 38, 12991–12999. 10.1016/j.ijhydene.2013.03.165. [DOI] [Google Scholar]
  29. Fan Y. J.; Zhu W.; Gong M.; Su Y.; Zhang H. W.; Zeng J. N. Catalytic gasification of dewatered sewage sludge in supercritical water: Influences of formic acid on hydrogen production. Int. J. Hydrogen Energy 2016, 41, 4366–4373. 10.1016/j.ijhydene.2015.11.071. [DOI] [Google Scholar]
  30. Gong M.; Zhu W.; Zhang H. W.; Ma Q.; Su Y.; Fan Y. J. Influence of NaOH and Ni catalysts on hydrogen production from the supercritical water gasifification of dewatered sewage sludge. Int. J. Hydrogen Energy 2014, 39, 19947–19954. 10.1016/j.ijhydene.2014.10.051. [DOI] [Google Scholar]
  31. Nanda S.; Dalai A. K.; Gokalp I.; Kozinski J. A. Valorization of horse manure through catalytic supercritical water gasifification. Waste Manage. 2016, 52, 147–158. 10.1016/j.wasman.2016.03.049. [DOI] [PubMed] [Google Scholar]
  32. Cheng Z.; Jin H.; Liu S. K.; Guo L. J.; Xu J. L.; Su D. Hydrogen production by semicoke gasification with a supercritical water fluidized bed reactor. Int. J. Hydrogen Energy 2016, 41, 16055–16063. 10.1016/j.ijhydene.2016.06.075. [DOI] [Google Scholar]
  33. Nanda S.; Dalai A. K.; Kozinski J. A. Supercritical water gasification of timothy grass as an energy crop in the presence of alkali carbonate and hydroxide catalysts. Biomass Bioenergy 2016, 95, 378–387. 10.1016/j.biombioe.2016.05.023. [DOI] [Google Scholar]
  34. Sheikhdavoodi M. J.; Almassi M.; Ebrahimi-Nik M.; Kruse A.; Bahramia H. Gasification of sugarcane bagasse in supercritical water; evaluation of alkali catalysts for maximum hydrogen production. J. Energy Inst. 2015, 88, 450–458. 10.1016/j.joei.2014.10.005. [DOI] [Google Scholar]
  35. Weijin G.; Binbin L.; Wang Q. Y.; Huang Z. H.; Zhao L. Supercritical water gasification of landfill leachate for hydrogen production in the presence and absence of alkali catalyst. Waste Manage. 2018, 73, 439–446. 10.1016/j.wasman.2017.12.015. [DOI] [PubMed] [Google Scholar]
  36. Xu D.; Wang S. Z.; Hu X.; Chen C. M.; Zhang Q. M.; Gong Y. M. Catalytic gasification of glycine and glycerol in supercritical water. Int. J. Hydrogen Energy 2009, 34, 5357–5364. 10.1016/j.ijhydene.2008.08.055. [DOI] [Google Scholar]
  37. Akgül G.; Kruse A. Influence of salts on the subcritical water gas shift reaction. J. Supercrit. Fluids 2012, 66, 207–214. 10.1016/j.supflu.2011.10.009. [DOI] [Google Scholar]
  38. Sınağ A.; Kruse A.; Schwarzkopf A. Key compounds of the hydropyrolysis of glucose in supercritical water in the presence of K2CO3. Ind. Eng. Chem. Res. 2003, 42, 3516–3521. 10.1021/ie030079r. [DOI] [Google Scholar]
  39. Zhu W.; Xu Z. R.; Li L.; He C. The behavior of phosphorus in sub- and supercritical water gasification of sewage sludge. Chem. Eng. J. 2011, 171, 190–196. 10.1016/j.cej.2011.03.090. [DOI] [Google Scholar]
  40. Sullivan P. A.; Tester J. W. Methylphosphonic acid oxidation kinetics in supercritical water. AIChE J. 2004, 50, 673–683. 10.1002/aic.10061. [DOI] [Google Scholar]
  41. Guo Y.; Wang S. Z.; Gong Y. M.; Xu D. H.; Tang X. Y.; Ma H. H. Partial oxidation of municipal sludge with activated carbon catalyst in supercritical water. J. Hazard. Mater. 2010, 180, 137–144. 10.1016/j.jhazmat.2010.04.005. [DOI] [PubMed] [Google Scholar]
  42. Weijin G.; Zhou Z. Z.; Liu Y.; Wang Q. Y.; Guo L. N. Hydrogen production and phosphorus recovery via supercritical water gasification of sewage sludge in a batch reactor. Waste Manage. 2019, 96, 198–205. 10.1016/j.wasman.2019.07.023. [DOI] [PubMed] [Google Scholar]

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