Phosphorus transformation in hydrothermal pretreatment and steam

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Biofuels and Biomass

Phosphorus transformation in hydrothermal pretreatment and steam gasification of sewage sludge Yuheng Feng, Kunyu Ma, Tianchi Yu, Shucheng Bai, Di Pei, Tao Bai, Qian Zhang, Lijie Yin, Yuyan Hu, and Dezhen Chen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01860 • Publication Date (Web): 26 Jun 2018 Downloaded from http://pubs.acs.org on June 27, 2018

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Phosphorus transformation in hydrothermal pretreatment and steam gasification of sewage sludge Yuheng Feng*, Kunyu Ma, Tianchi Yu, Shucheng Bai, Di Pei, Tao Bai, Qian Zhang, Lijie Yin, Yuyan Hu and Dezhen Chen Thermal and Environmental Engineering Institute, School of Mechanical Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China *Corresponding author Yuheng Feng, Thermal and Environmental Engineering Institute, Tongji University, 200092, Shanghai, China Tel.: +86 21 65985009; Fax: +86 21 65982786 E-mail address: [email protected]

Abstract Sewage sludge is an important pool of phosphorous (P), while thermal disposal results the enrichment of P into final solid product. Hydrothermal treatment combined with steam gasification is an effective method to produce high-quality syngas from sewage sludge. This study discussed the transformation of P during this integrated thermochemical process based on thermodynamic calculation and SMT analysis of solid products. The results showed that P was enriched in hydrochar with the concentration above 14mg/g. By the pretreatment, OP was significantly removed from solid due to destruction while IP was mostly recovered. Part of OP was converted to IP in disposal from 220oC to 240oC. At 260oC, OP was emitted to gaseous phase thus not captured by minerals. In addition, NAIP tend to be transformed into AP with the increase of pH and CaO addition significantly promoted this transformation. In gasification process, OP was completely removed while IP was almost recovered with the substantial conversion from NAIP to AP. The highest total P recovery (84.92%) was achieved from GA-200-CaO while P was the most concentrated in GA -240 (30.39mg/g). This study provided basic data for P recovery from sewage sludge using thermochemical conversion. 1. Introduction Among the traditional treatments of Sewage sludge (SS), landfill and incineration might cause secondary pollution and anaerobic digestion has a long processing period1, so gasification was introduced as a novel method with less secondary pollution and shorter processing time2. Meanwhile, hydrogen-rich syngas could be produced when using steam as the gasification agent3. The high moisture content in SS calls for effective and economical dehydration method before its

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thermal conversion. Hydrothermal treatment highly enhances mechanical dewaterability of sludge with low energy cost4. Besides, it also reduces N emission and improves syngas quality during steam gasification5, 6. Despite producing high quality hydrogen-rich syngas, as a thermochemical disposal method, gasification results in a highly concentrated P content in sludge ash7. P is an essential element for all living organisms. Phosphate rock is the primary source of P fertilizer while it will be depleted in one century8. In National mineral resource planning issued by Ministry of Natural Resources in China (2016-2020), P was listed in the category of strategic minerals. SS produced from water treatment is rich in P. In China, there was approximate 125,000 tons of P out of 24.186 million tons of SS production in 20129. Furthermore, P concentration in sludge is still increasing with the improvement of the water treatment technology10. Therefore, it’s promising to develop P recovery technology from SS, which is recognized as the second largest source of P7. Generally, P in SS is classified into organic and inorganic. The inorganic P could be further classified based on different bonding abilities11. Not only the total P in materials its recovery, the P forms affects its availability and decide the final extraction method. For instance, NAIP (P which are associated with oxides and hydroxides of Al, Fe, and Mn) is potentially active and its instability and solubility leads to P loss during the thermal conversion. AP (Ca associated P) with less bioavailability is immobilized in the conversion and can be recycled in a bioavailable phosphate form12. P migration and transformation during thermochemical conversion of SS is widely focused on in recent studies. Qian13 examined the migration of phosphorus in SS during different thermal treatment processes and the results showed that temperature significantly influences the species and content of P in the sewage sludge char or ash. Huang14, 15 used 31P nuclear magnetic resonance (NMR) and P K-edge X-ray absorption near-edge structure (XANES) to compare P transformations during hydrothermal and pyrolysis process. Wang16 found that the variation of feedwater pH stimulated P transformation during hydrothermal carbonization of SS. However, the effect of hydrothermal condition on P recovery and speciation in the solid product of subsequent thermochemical process was not referred in these studies. In this study, we focused on the hydrothermal temperature on the recovery and speciation of P in steam gasification ash of hydrochars. Since lime is widely used in sludge conditioning and stabilization processes17, the effect of CaO addition before the treatment was also discussed. Hydrochars produced in a sub-critical hydrothermal reactor at 200-260oC were gasified in steam atmosphere. Contents of different P forms in raw SS, hydrochars and gasification ash were examined according to Standards, Measurements and Testing Programme (SMT) protocol. The conversion mechanism of P during the treatment was discussed with the help of minimum free Gibbs method. The result provides basic data for appropriate pre-treatment condition targeting


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recycling of P from steam gasification of SS. 2. Materials and methods 2.1. Materials The activated sludge with 80.76% moisture content used in this study was from a municipal waste water treatment plant in Shanghai. The sludge was treated in a 2L subcritical hydrothermal reactor at 200oC, 220oC, 240oC and 260oC for 0.5h. The hydrochars obtained were named as HT-200, HT-220, HT-240 and HT-260. To examine the effect of CaO addition on phosphorous transformation, 10% of CaO (based on dried sample) was mixed with raw wet sludge and treated at 200oC and 260oC. The char produced was named as HT-200-CaO and HT-260-CaO. The dried raw sludge and hydrochars were gasified in steam atmosphere in a fixed-bed batch reactor at 900oC, with the steam/hydrochar (or sludge) mass ratio 2.4:1. The ash obtained from gasification of raw sludge were named as GA-SS and those from hydrochars were named as GA-200 and so on. The detailed description of hydrothermal reaction, gasification reaction and the elemental and proximate analysis was in previous report. The proximate and ultimate analysis of HT-200-CaO are listed in Table 1 while the analysis of raw sludge and other hydrochars were also presented in our previous study18. The solid yield of HT-200-CaO was 91.69% based on dried raw sludge. Moreover, Table 2 lists the relative content of the element in inorganic phase of raw dreid sludge and hydrochars was semi-quantified by an XRF analyzer. Table 1. Proximate and ultimate analysis of HT-200-CaO (%) VMa

FCb

Ash

C

H

O

N

46.34

2.30

50.96

27.83

4.23

14.64

2.34

a

Volatile matter

b

Fixed carbon Table 2. relative content of the element in inorganic phase of dried sludge and hydrochars (%) Sample

Mg

Al

Si

K

Ca

Ti

Fe

Dried sludge

0.39

2.03

5.21

0.90

4.24

0.37

8.97

HT-200

0.66

3.27

7.36

0.95

5.26

0.43

9.40

HT-220

0.85

4.00

9.16

1.02

5.82

0.46

9.32

HT-240

0.91

4.09

9.52

1.04

6.51

0.45

9.82

HT-260

0.92

4.23

9.71

1.06

6.82

0.45

10.51

HT-200-CaO

0.73

3.98

8.61

0.83

15.36

0.38

7.36

HT-260-CaO

0.88

3.08

8.90

0.85

18.21

0.38

8.82


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2.2. P fraction in solid sample SMT protocol was used to extract different fractions of P in SS, hydrochar and gasification ash, including total phosphorus (TP), organic phosphorus (OP), inorganic phosphorus (IP), NAIP (which are associated with oxides and hydroxides of Al, Fe, and Mn), and AP (Ca associated forms). The relation among these fractions are TP=OP+IP and IP=AP+NAIP. Different fractions of P extracted by corresponding solution was quantified by an SPS5520 inductively coupled plasma mass spectrometry (SII Nanotechnology, Japan). The concentration of different P fractions in dried raw sludge is displayed in Table 2. Table 3. Concentration of different P fractions in dried raw sludge by SMT (mg/g) TP

OP

IP

NAIP

AP

10.24

2.4

7.87

4.27

3.68

2.3 Thermochemical equilibrium method Thermochemical equilibrium method was used to predict the transformation of P fractions in raw sludge. The calculation was performed by HSC-chemistry 6.1, which is based on minimization of the Gibbs free energy. To analyze the thermal behavior of phosphorus oxide from OP destruction, conversion of 1mol P2O5 with temperature in inert atmosphere at 20bar was examined. As to NAIP conversion, AlPO4 was used as the model compound and its amount was set as 1mol. An idealized AlPO4-water system was adopted. The water amount was set according to the ratio of moisture to NAIP content in raw wet sludge. The hydrothermal treatment promoted the uniform distribution of sludge matrix and additives in aqueous phase and provide high temperature for the reactants to overcome the energy barriers. After the treatment, the final product was obtained at ambient environment. Therefore, the equilibrium condition was set as room temperature (20oC) and environment (1bar). To investigate the conversion of NAIP in basic environment, NaOH was added to the AlPO4-water system with NaOH/P ratio from 0 to 3.0. On the other hand, CaO/P ratio varied from 0 to 2.5 to examine the effect of CaO addition. 3. Results and discussions 3.1 P speciation during hydrothermal pretreatment and gasification process 3.1.1 pH analysis of hydrothermal liquid pH value of liquid in the hydrothermal reactor was an important factor that affect the distribution of different P fractions in char16. The pH value of raw sludge and hydrothermal liquid accompanied with hydrochars was listed in Table 2. It could be found that the liquid phase of raw sludge was almost neutral (pH=7.01) while it was raised to 7.20 after the treatment at 200oC and continuously increased with the temperature. The pH of liquid accompanied with HT-260 was 8.23. Deamination reaction of the proteins occurred for in hydrothermal condition18, leading to


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the release of NH3. In addition, acidic compounds volatilization19 also contributed to the pH value increase. Nevertheless, the addition of CaO before treatment at 200oC elevated the pH value of hydrothermal liquid to 8.01 since the Ca(OH)2 formation and its solution in liquid. However, addition of CaO at 260oC slightly decreased the pH of liquid product. CO2 was among the main the devolatilization product during the treatment of SS. There’re probably two reasons for the pH decrease. In the first place, CaO addition at 260oC had a more effective promotion effect on the devolatilization of sludge matrix compared with that at 200oC. This was corresponding with the obvious difference of hydrochar yield based on dried sludge, which was 91.69% for HT-CaO-260 and 65.91% for HT-CaO-2006. On the other hand, CO2 absorption rate by Ca(OH)2 solution was raised by temperature increase. This could be evidenced by XRD pattern of HT-260-CaO in Fig 1, in which the corresponding peaks of CaCO3 was much more apparent compared those of Ca(OH)2. Therefore, significantly more CO2 solved led to the pH adjustment of hydrothermal liquid with HT-260-CaO. Table 4. pH value of raw sludge and hydrothermal liquid accompanied with hydrochars Raw sludge

HT-200

HT-220

HT-240

HT-260

HT-200-CaO

HT-260-CaO

7.01

7.20

7.67

7.77

8.23

8.01

8.20

2500

2000

Intensity(Counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1500

1000

500

0 Quartz, syn - SiO2

Calcite - CaCO3

Portlandite, syn - Ca(OH)2

10

20

30

40

50

60

70

Two-Theta (deg)

Fig 1. XRD pattern of HT-CaO-200

3.1.2 SMT analysis of hydrochars The P concentrations and recoveries of dried sludge and hydrochars were in Fig 2. The recovery rate was over 80% for all the hydrothermal conditions while the highest rate when no CaO added


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was found in HT-220 (93.82%). The P concentrations was effectively raised by the treatment since the high recovery and the significant removal of volatile species6. Raising the treatment temperature from 200oC to 220oC led to a substantial increase of P concentration in hydrochar, from 14.01mg/g to 16.10mg/g. Above 220oC, there was a slight increase and the concentration was 16.21mg/g for HT-260. The addition of CaO into raw sludge before treated at 200oC raised the P recovery to 94.29%. On the other hand, the addition before treating at 260oC decreased the P recovery from 82.27% to 80.63%. P concentration was reduced by the CaO addition in both temperatures. Concentration(mg/g) Recovery(%)

18

100

15

80

12 60 9 40 6

Recovery(%)

Concentration(mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20

3

0

0 HT-200

HT-220

HT-240

HT-260

HT-200-CaO

HT-260-CaO

Fig 2. P concentrations and recoveries of raw sludge and hydrochars To analyze the conversion of different fractions of P during the treatment, the recoveries of different P fractions in hydrochars are displayed in Table 3. Due to the low boiling point of OP20, its recovery was obviously lower than the IP fractions, which was 42.79% when treated for HT-200 and even lower at other conditions. Niu21 selected C18H15O4P as the model P compound and tested its thermal stability by TGA. The result showed that the decomposition initiated at 160oC. IP recovery of HT-220 and HT-240 was above 100%, indicating that a certain proportion of OP was transformed into IP. Zhu22 found that OP was mostly converted into IP during hydrothermal disposal of SS. Highest IP recovery was found at 220oC (100.76%) and decreased to around 90% at higher temperature. On the other hand, AP was continuously increased from 82.76% at 200 oC to 131.72% at 240oC, then sharply decreased to 100.98% at 260oC. Wang16


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found that NAIP tended to be transformed into AP when raising pH of feedwater in hydrothermal reactor. In addition, both AP and NAIP concentrations were decreased from 230oC to 260oC in alkali environment. For both hydrochars with CaO addition, OP recovery was decreased compared with the corresponding chars with no CaO added since the promotion effect of CaO on destruction of organic proportion. The decrease was more apparent for HT-CaO-260 since the promotion of devolatilization by CaO addition was more effective at 260oC. Moreover, there was a significant NAIP reduction with AP increase, indicating the conversion from NAIP to AP as a result of the interaction of added CaO with P. This tendency was also found during the heating of mixed SS ash and CaO10 from 750oC to 950oC. Table 3. Recovery of different P fractions in hydrochars (%) Sample

OP

IP

NAIP

AP

HT-200 HT-220 HT-240 HT-260 HT-200-CaO HT-260-CaO

42.79 39.28 34.81 36.16 38.97 23.89

91.49 110.09 110.11 96.02 110.80 97.63

97.17 100.76 89.69 90.31 25.55 11.88

82.76 119.18 131.72 100.98 207.80 195.35

3.1.3 SMT analysis of gasification ash The recoveries and concentrations of IP, NAIP and AP in gasification ash of dried sludge and different hydrochars was in Table 4. P was highly concentrated in ash, with the value over 20mg/g in ash for all the feedstocks. Since the high P recovery and the removal of soluble minerals during hydrothermal treatment, P concentration in ash from hydrochars was higher than that in dried sludge, with highest concentration found in GA-240 (30.39mg/g). Table 4. Recovery and concentration of different P fractions in gasification ash Sample Dried sludge ash GA-200 GA-220 GA-240 GA-260 GA-200-CaO GA-260-CaO

Recovery (%)

Concentration (mg/g)

IP

NAIP

AP

IP

NAIP

AP

106.91 98.03 91.83 99.64 99.95 99.93 105.92

8.93 6.77 8.77 10.39 11.23 30.43 36.59

220.60 222.96 172.28 169.86 192.22 109.65 110.73

21.63 27.91 29.95 30.39 29.48 23.88 21.48

0.98 1.11 1.42 1.40 1.69 0.91 0.49

20.87 26.85 28.44 28.98 27.88 22.98 21.01


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No OP was detected ash since its complete devolatilization at high temperature21. Meanwhile, the recoveries of IP for all the samples were over 95%, indicating the minor IP loss in the steam gasification process. Besides, it could be found that the IP recoveries in sludge ash and GA-CaO-260 were even higher than 105%. To explain this phenomenon, the surface area of dried sludge and hydrochars are displayed in Table 5, showing that hydrothermal treatment increased the surface area of sludge while CaO addition before treatment decreased it probably due to the pore blockage by diffusion of additives. Moreover, HT-260-CaO has a significantly lower surface area compared with HT-200-CaO which was caused by the promotion of additive diffusion when temperature raised. The relative lower surface area of dried sludge and HT-260-CaO lowered the release rate of phosphorus oxide from OP destruction into gaseous phase during heating, thus enhanced its possibility to interact with mineral phase and form IP. Li10 examined the P speciation with incineration temperature of SS, the result showed that AP was declined slightly from 450oC to 850oC while decreased suddenly from 850oC to 950oC. Conversely, NAIP was increased slightly from 450oC to 950oC. However, here in steam atmosphere, apparent conversion from NAIP to AP occurred in the heating process, with the highest conversion rate (93.25%) found for GA-200. Therefore, steam effectively promoted the transformation of NAIP to AP in SS and hydrochars during the heating process. The lowest NAIP loss rate was found for GA-260-CaO, since the abundant transformation of NAIP to AP already occurred in hydrothermal treatment. Due to the high conversion rate of NAIP to AP during pretreatment or steam gasification, AP took higher than 94% of TP in ash from all the feed stocks. Table 5. Surface area of dried sludge and hydrochars* (m2/g) Dried sludge

HT-200

HT-260

HT-200-CaO

HT-260-CaO

3.78

37.22

36.87

11.20

3.82

* Data of dried sludge, HT-200 and HT260 were also in previous literature6 The P recovery during gasification process and whole process including pretreatment and gasification were displayed in Fig 3. For dried sludge, although a certain fraction of OP was converted to IP and reserved in ash during gasification, its P recovery was the lowest due to the fraction of OP released. The gasification recovery was raised with the treatment extent. This was due to the decrease tendency of OP proportion in hydrochars with treatment extent, which was easily released into gaseous phase during gasification. The highest recovery was achieved by


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GA-260-CaO (99.35%) since the substantial capture of OP devolatilized. Due to the effective conversion of OP into IP during hydrothermal treatment, the total P recovery in GA-220, GA240 and GA-200-CaO were higher than dried sludge ash in spite of the P loss in the treatment. The highest recovery was found in GA-200-CaO, with 84.92% of P recovered.

100

Gasification recovery Total recovery

90

Recovery(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

70

60 Dried sludge ash GA-200

GA-220

GA-240

GA-260

GA-200-CaO GA-260-CaO

Fig 3. P recoveries and concentrations in gasification ash of dried sludge and hydrochars

3.2 Analysis of P conversion during hydrothermal treatment by thermodynamic equilibrium method 3.2.1 Conversion of P2O5 in heating process P released during heating of OP is often in the form of phosphorus oxides23, so the conversion of P2O5 with temperature was examined by minimum free Gibbs energy method. The result is displayed in Fig 4. It could be found that at low temperature, P-O is in the form of solid P2O5. Between 250oC and 260oC, solid and liquid P starts to vaporize as P4O10.

2PO 2 5 (s) →PO 4 10 (g) (1) At 556oC, all P in the system was converted to P4O10. This is corresponding to the results in SMT analysis, in which IP recoveries for HT-260 and HT-260-CaO were both below 100%, largely reduced compared with HT-240 and HT-CaO-200 respectively. This indicated that the interaction of P compounds from OP destruction with the minerals in sludge matrix was inhibited due to the escape of P4O10 into gaseous phase.


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P2O5(s) P4O10(g) 1.0

Amount of substance (mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8

0.6

0.4

0.2

0.0 0

200

400

600

800

Temperature

Fig 4. Conversion of P2O5 with temperature at 20bar 3.2.2 Conversion of NAIP in basic environment According to the pH test, basic liquid was produced during hydrothermal treatment. Hence the conversion of NAIP with alkali addition is worth discussing since its instability in basic environment. Wang16 discussed the P conversion with pH value. However, the detailed conversion path of P compounds was not revealed. In addition, since the IP concentration was less than 2% in hydrochars, the IP compounds was hardly identified in XRD pattern as in Fig 5. So minimum free Gibbs energy method was used to calculate the conversion of Al/P products and pH with NaOH addition into AlPO4-water system and the results are displayed in Fig 2. Although AlPO4 is an insoluble mineral, the hydrolysis of the minute quantity resulted the acidity of aqueous phase. The addition of NaOH into water causes the intermediate increase of pH value, exceeding 7.0 at NaOH/P=0.25. With the further addition of NaOH, the pH value is mildly raised, to 7.4 at NaOH/P=1.62. This indicates that the existence of NAIP has a cushioning effect on pH increase when alkali is added into the aqueous phase due to reaction 2.

AlPO4 + 3OH − → Al(OH )3 +PO43−

(2)

Meanwhile, the hydrolysis of PO43- lead to the formation of more OH-, which enhances the Al(OH)3 precipitation.

PO43− +H2O → HPO42− + OH −

(3)

HPO42− +H2O → H2 PO4− + OH −

(4)

AlPO4 amount shows an almost linear decrease with NaOH amount. When NaOH/AlPO4=1.62, all AlPO4 is converted to Al(OH)3. With excessive NaOH added, reaction 4 tends to proceed the reverse direction with the sharp increase of pH value. Thus, HPO42- formation is accelerated until


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reaching highest quantity (0.99mol) at NaOH/P=2. Then PO43- generates in the aqueous phase due to the inhibition of its hydrolysis (reaction 3) accompanied with further pH increase, while HPO42- tendency is in the opposite direction. The increase of pH value gradually become gentle due to its logarithmic relation with OH- concentration. The calculation indicates that AlPO4 tends to be solved in basic aqueous phase and form HPO42- and PO43-, which are sources of AP precipitation including Ca(HPO4)2 and CaPO4. This corresponds to the tendency found in SMT analysis, in which NAIP was converted to AP with the pH increase. Al(OH)3 2HPO4 AlPO4 PO43H2PO4 pH

1.0

0.8

14

12

0.6

10

pH

Amount of substance(mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4 8 0.2 6 0.0 4 0.0

0.5

1.0

1.5

2.0

2.5

3.0

NaOH/P (-)

Fig 5. Evolution of Al/P products and pH with NaOH addition into AlPO4-water system 3.2.3 Conversion of NAIP after CaO addition CaO was widely used as the conditioner during water treatment and its addition to SS also raised the H2 production during gasification process. Here the transformation of NAIP with addition of CaO was investigated by minimum Gibbs free energy method. The calculation result in Fig 6 shows that Ca5(PO4)3OH was the dominant Ca-P compounds forms during the addition of CaO, with almost 1/5 mole number of CaO added. Besides, there is minor formation of Ca2+ and H2PO42- during the initial addition. All P is converted to Ca5(PO4)3OH at Ca/P=1.66. Then Ca2+, CaOH+ and OH- formed and their quantity increases due to the solution of Ca(OH)2. When Ca/P was above 2.05, excessive Ca(OH)2 kept in condensed phase since it’s slightly soluble in water. This result evidenced the findings in SMT analysis of sludge and hydrochars with CaO addition, in which significant amount of NAIP was transformed into AP by adding CaO before the treatment.


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Al(OH) 3 AlPO4 OHCa(OH) 2 Ca5(PO 4) 3OH Ca2+ CaOH+ H3PO4

1.0

Amount of substance (mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8

0.6

0.4

0.2

0.0 0.0

0.5

1.0

1.5

2.0

2.5

Ca/P(-)

Fig 6. Evolution of Al/Ca/P compounds with CaO addition into AlPO4-water system

4. Conclusion This study discussed the transformation of P during the integrated thermochemical process combining hydrothermal treatment and steam gasification. The results showed that OP was significantly removed from solid during the treatment due to destruction while IP was mostly recovered. Part of OP was converted to IP in disposal from 220oC to 240oC. At 260oC, OP was emitted to gaseous phase thus not captured by minerals. Meanwhile, NAIP tend to be transformed into AP with the increase of pH and CaO addition significantly promoted this transformation. In gasification process, OP was completely removed while IP was almost recovered with the substantial conversion from NAIP to AP. The highest total P recovery (84.92%) was achieved from GA-200-CaO while P was the most concentrated in GA -240 (30.39mg/g). This study showed that OP could be effectively converted into IP during hydrothermal pretreatment of SS with suitable condition, which increases the total P recovery of the whole thermochemical conversion. In addition, immobilized AP was the dominant P fraction in the gasification ash from the sludge and its hydrochars. Acknowledgements This work was supported by National Natural Science Foundation of China [grant number


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Energy & Fuels

51706156], Shanghai Rising-Star Program [grant number 17QC1401000], CAS Key Laboratory of Renewable Energy [grant number Y707k41001] and Visiting Scholar foundation of Key Lab of Clean Energy Utilization in Zhejiang University [grant number ZJUCEU2015017].

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Phosphorus transformation in hydrothermal pretreatment and steam

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