Effect of Hydrothermal Treatment on the Steam Gasification Behavior

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Effect of Hydrothermal Treatment on the Steam Gasification Behavior of Sewage Sludge: Reactivity and Nitrogen Emission Yuheng Feng,* Tianchi Yu, Dezhen Chen, Genli Xu, Lu Wan, Qian Zhang, and Yuyan Hu Thermal and Environmental Engineering Institute, School of Mechanical Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, People’s Republic of China ABSTRACT: Hydrothermal treatment not only reduces the cost of moisture removal but also improves the quality of the syngas from steam gasification for sewage sludge. However, the effect of the treatment on the gasification behavior, including the reactivity and emission of gaseous N compounds, was not clear. In this study, the kinetic analysis based on a thermogravimetric (TG) experiment was used to examine the reactivity of the hydrochars. In addition, the evolution of N functionalities in hydrochar with treatment conditions and the discharge of NH3 and HCN during gasification were also investigated. The result showed that the treatment lowered the activation energy of sludge in gasification of remaining char and effectively reduced the discharge of NH3 and HCN. CaO addition into raw sludge before treatment caused the shift of the peak temperature to the lower range in remaining char gasification and promoted the conversion of HCN to NH3.

1. INTRODUCTION According to the report from the National Bureau of Statistics, the annual yield of sewage sludge in China had approached 36 million tons in 2014, which will cause heavy pollutants if not disposed of properly. Landfill, incineration, and anaerobic digestion are the traditional treatments for sewage sludge. However, landfill and incineration might cause secondary pollution, and anaerobic digestion has a long processing period.1 Therefore, gasification was introduced as a novel method with less secondary pollution and shorter processing time.2 Steam is a promising gasification agent as a result of the high hydrogen content in syngas.3 The high moisture content in sewage sludge will lead to high energy costs for the dehydration before the gasification process.4 Hydrothermal treatment is an effective way to enhance the dehydration efficiency. Escala et al.5 found that, after hydrothermal treatment, the mechanical dewaterability was significantly increased for both stabilized and non-stabilized sludge. In addition, hydrothermal treatment also improves the syngas quality. Moon et al.6 pretreated sewage sludge in a hydrothermal reactor before steam gasification and obtained higher methane and hydrogen yields. By examination of the difference of the physichemical characteristics between raw sewage sludge and its hydrochars, Gai et al.7 found that the hydrochar was rich in hydrophilic functional groups and catalytic metal components, which favored the higher syngas yield and hydrogen production. Besides the syngas quality, the gasification reactivity of the hydrochars will also affect its industrial application performance. Gasification in steam could be divided into two steps, including pyrolysis and the subsequent gasification of the remaining char. The latter step is the controlling step of the overall process.8 Hence, the reactivity of the remaining char during high-temperature gasification is fundamental knowledge in the study of gasification because it affects the residence time of the feeding material in the reactor. Kinetic parameters could reveal the reactivity of the sample in the reaction. A thermogravimetric (TG) experiment was usually used for the © XXXX American Chemical Society

kinetic analysis of the fuels during combustion, gasification, and pyrolysis.9−13 In the syngas from steam gasification of sewage sludge, there are N-containing compounds, such as NH3 and HCN, which are the precursors of NOx and NO2 in combustion.14 Although they could be removed before the combustion turbines, it will increase the capital and maintenance costs of power generation.15 A significant amount of nitrogen in sewage sludge could migrate into the liquid phase during hydrothermal treatment, which provide an opportunity to upgrade the fuel quality.16 Recently, there has been growing interest in the nitrogen transformation pathways during the hydrothermal reaction of sewage sludge and its impact on the subsequent combustion. He et al.17 investigated the different regimes for the deamination reaction in the hydrothermal treatment of sewage sludge. Zhao et al.18 compared the NO discharge from raw sludge and hydrochar in different combustion conditions and found that hydrothermal treatment effectively lowered the discharge. Nevertheless, there are few studies on the nitrogen emission behavior during steam gasification of hydrochars from sewage sludge. Lime was usually used as the conditioner during sewage sludge disposal.19 It not only improves the syngas quality of steam gasification20 but also inhibits the formation of toxic pollutants.21 The high temperature and turbulent environment could promote the uniform distribution of lime on the solid phase of the sludge matrix. Hence, it is necessary to examine the effect of CaO addition before hydrothermal treatment on the hydrochar reactivity and nitrogen emission during steam gasification. This study focused on the effect of hydrothermal treatment on the reactivity and nitrogen emission of hydrochars during steam gasification. Activated sewage sludge was treated in the Received: October 26, 2017 Revised: November 24, 2017 Published: December 1, 2017 A

DOI: 10.1021/acs.energyfuels.7b03304 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Proximate and Ultimate Analyses of the Dried Raw Sludge (wt %)

a

sample

VMa

FCb

ash

N

C

H

S

O

raw sludge

59.24

3.86

36.9

4.13

31.29

5.15

3.25

19.28

Volatile matter. bFixed carbon.

Figure 1. Schematic of the hydrothermal treatment of the sewage sludge: (1) plunger, (2) pressure gauge, (3) inside thermocouple, (4) surface thermocouple, (5) controller, (6) hydrothermal reactor, (7) heating wire, (8) sewage sludge, (9) sludge outlet, (10) water bath, and (11) flowmeter. was heated in an α-Al2O3 crucible from 150 to 950 °C at the rate of 8 K/min and then kept at 950 °C for 30 min. Steam (5 g/h) was purged into the furnace by argon steam (20 mL/min). The flow rate of the shielding gas (argon) was 20 mL/min. The kinetics of each region during steam gasification could be described as

range from 200 to 260 °C in a subcritical hydrothermal reactor with or without CaO pre-added. The reactivity of hydrochars in steam gasification was discussed using kinetic analysis based on TG experiments. To reveal the reduction of nitrogen emission, both the conversion of N functionalities in a sludge matrix with treatment conditions and the discharge of gaseous Ncontaining products in steam gasification of the hydrochars were examined. The results will provide basic knowledge for the optimization of the hydrothermal treatment conditions of sewage sludge targeting better steam gasification performance.

dα = kf (α) dT

(1)

where α and k represent the reacted mass fraction and rate constant, respectively, and T is the thermodynamic temperature in kelvin. When the rate constant is in Arrhenius form

2. MATERIALS AND METHODS

⎛ E ⎞ ⎟ k = A exp⎜ − ⎝ RT ⎠

2.1. Materials. The raw sludge used in this study was the activated sludge obtained from a municipal water treatment plant in Jiading District, Shanghai, China. The water content in raw sludge was 80.76%. The proximate and ultimate analyses of the dried sample were in Table 1. 2.2. Hydrothermal Reaction. The raw sludge was treated in a subcritical hydrothermal reactor, and the schematic is in Figure 1. The reaction temperature was set at 200, 220, 240, and 260 °C. To study the effect of CaO addition on steam gasification, it was mixed with the raw wet sludge and treated at 260 °C, for the best interaction of the additive and sludge matrix. The residence time for the sludge at the final temperature was 0.5 h. When the reactor was cooled to room temperature, the gaseous products were vented. The mixture of char and liquid left in the reactor was centrifuged at 6000 revolutions/min for 10 min. The wet char separated was dried at 105 °C for 10 h. The dried char was named char-200, char-220, char-240, and char-260, while the char with CaO pre-added was named char-260-CaO. 2.3. TG Experiment and Kinetic Analysis. The weight loss behavior of dried sludge, char-200, char-260, and char-260-CaO during steam gasification was studied by a thermogravimetric analyzer (NETZSCH STA 449 F3 Jupiter). A total of 10 mg of the sample

(2) −1

−1

where R is the universal gas constant (R = 8.314 J mol K ), E is the activation energy, and A is the frequency factor. In a non-isothermal reaction with the constant heating rate β, eq 2 could be transformed into

dα k = dT f (α) β

(3)

After integration, eq 3 is represented as g (α) =

∫0

α

dα A = f (α) β

∫0

T

⎛ E ⎞ ⎟ dT exp⎜− ⎝ RT ⎠

(4)

in which g(α) is the integral form of the kinetic mechanism function. Using the Coats−Redfern method22 for integration of eq 4

ln

g (α) T

2

⎡ AR ⎛ 2RT ⎟⎞⎤ E = ln⎢ ⎜1 − ⎥− ⎣ βE ⎝ E ⎠⎦ RT

(5)

If the term 2RT/E is much less than 1, it could be neglected. Hence B

DOI: 10.1021/acs.energyfuels.7b03304 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 2. Different Mechanism Functions Used in the Calculation of Kinetic Parameters

ln

mechanism

integral form, g(α)

differential form, f(α)

P1, power law P2, power law P3, power law P4, power law A1.5, Avrami−Erofeev A2, Avrami−Erofeev A3, Avrami−Erofeev A4, Avrami−Erofeev R2, contracting surface R3, contracting volume D1, 1D diffusion D2, 2D diffusion D3, 3D diffusion 1.5 D4, Ginstling−Brounshtein F1, first order F2, second order

1 2α1/2 3α2/3 4α3/4 1.5(1 − α)[−ln(1 − α)]1/3 2(1 − α)[−ln(1 − α)]1/2 3(1 − α)[−ln(1 − α)]2/3 4(1 − α)[−ln(1 − α)]3/4 2(1 − α)1/2 3(1 − α)2/3 1/2α −[ln(1 − α)]−1 [1 − (1 − α)4/3]−1(1 − α)2/3 − 1 1.5[(1 − α)−1/3 − 1]−1 1−α (1 − α)2

α α1/2 α1/3 α1/4 [−ln(1 − α)]2/3 [−ln(1 − α)]1/2 [−ln(1 − α)]1/3 [−ln(1 − α)]1/4 1 − (1 − α)1/2 1 − (1 − α)1/3 α2 (1 − α)ln(1 − α) + α [1 − (1 − α)1/3]−2 (1 − 2α/3)−1/3 − (1 − α)2/3 −ln(1 − α) (1 − α)−1 − 1

g (α) T

2

⎛ AR ⎞ E = ln⎜ ⎟ − ⎝ βE ⎠ RT

(6)

The activation energy E and frequency factor A could be decided by linear fitting with the slope −E/R and intercept ln(AR/βE) from the data of T and α obtained from TG analysis. For each reaction region, all of the functions in Table 2 were tried and the function with the highest determination coefficient (r2) was selected. 2.4. N Functionalities and Gaseous N-Containing Product Analysis. X-ray photoelectron spectroscopy (XPS) analysis was used to semi-quantify the different N functionalities in dried raw sludge, char-200, char-260, and char-260-CaO. The experiment was performed on an ESCALAB 250 Xi spectrometer with an Al Kα Xray source (20 kV and 10 mA). The spectrometer was operated under the pass energy of 20 eV for the survey spectra and 100 eV for the single-element spectra. All samples were analyzed under identical conditions and referenced to the C 1s peak at 284.6 eV. Least squares curve fitting of the spectra were performed using Gauss−Lorentzian shapes. The relative content of each N functionality was normalized from dividing the corresponding peak area value by the total value of the peaks in the N 1s XPS spectra of raw sludge. The release characteristics of gaseous N-containing products of the raw sludge and hydrochars were investigated using a tube furnace connected to a portable FTIR analyzer GASMET Dx-4000 (Temet Instrument Oy, Finland). The ceramic boat containing 0.5 g of the sample was placed in the middle of the furnace and heated from 150 to 850 °C at the rate of 8 °C/min and then kept at 850 °C for 20 min until the contents of all of the gaseous N were less than 10 ppm. The steam at 0.5 g/min was purged into the furnace by N2 at 4 L/min. A ceramic filter heated to 150 °C was set before the inlet of the FTIR analyzer to trap the tar. All of the pipes connecting the furnace and analyzer were heated to 150 °C to avoid the condensing of the gaseous N compounds.

Figure 2. Moisture content and yield of the hydrochars in different treatment conditions.

wet char-200. Raising the treatment temperature also favored the moisture removal. For char-260, the moisture content was only 57.1% of the solid yield. The solid yield for char-200 was 58.6% as a result of the loss of volatile matter (VM) during the treatment. There was a slight increase of the yield from 200 to 220 °C, while a decrease at higher temperatures. CaO was usually used as a conditioner to raise the dehydration efficiency of sewage sludge;23 therefore, when it was pre-added before the treatment at 260 °C, the water content in the mechanically dehydrated char was decreased. The pH of the liquid was detected, and the value was 9.37. This basic environment of the aqueous phase meant the formation of Ca(OH)2 after CaO addition. During the treatment, CO2 emitted from devolatilization could be captured by Ca(OH)2 and form Ca(CO3)2.24 These two calcium products migrated to the ash of char-260CaO after drying, leading to the increase of the solid yield. 3.1.2. Chemical Composition. The proximate and elemental analyses of the dried sludge and hydrochars were listed in Table 3. It could be found that the hydrothermal treatment significantly reduced the VM in the sample, from 59.24% in dried sludge to 50.14% in char-200 and further reduced to 41.95% in char-260. The fractions of ash and fixed carbon (FC) were increased as compensation. The fraction of the sludge FC retained in char-200 was 70.0%, while it was 96.4% for char-

3. RESULTS AND DISCUSSION 3.1. Hydrothermal Treatment on the Physiochemical Properties of Char. 3.1.1. Solid Yield Water Content. The moisture contents of the wet hydrochars after centrifugation and the solid yield based on dried raw sludge were displayed in Figure 2. The gas production from the treatment was not discussed here because it is less than 0.5 wt % of the raw wet sludge in all conditions. It could be found that the dehydration efficiency of the mechanical method was significantly increased by the treatment at 200 °C. A total of 74.6 wt % of the raw sludge was removed after the treatment and subsequent centrifugation, resulting in 66.8% of the moisture content in C

DOI: 10.1021/acs.energyfuels.7b03304 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 3. Proximate and Ultimate Analyses of the Hydrochars (wt %) sample

VM

FC

ash

N

C

H

S

O

char-200 char-220 char-240 char-260 char-260-CaO

50.14 43.85 43.54 41.95 41.42

4.60 6.32 6.45 7.16 1.23

45.26 49.83 50.01 50.89 57.35

3.20 2.57 2.36 2.43 1.79

31.78 30.22 29.69 30.73 28.01

4.7 4.17 4.17 4.23 3.83

3.49 4.01 3.87 3.77 2.45

11.57 9.19 9.91 7.95 6.57

be clearly divided into two stages, including devolatilization and steam gasification of remaining char. The dividing points of the two stages were all around 550 °C. The weight loss decreases in the first stage while increases in the second stage with the hydrothermal extent. This coincided with the fractions of VM and FC in the proximate analysis of these samples. With reference to the DTG curve of char-260, the heating process of char-260-CaO in a steam atmosphere could also be divided into two stages, with the critical point at 532 °C. Part of Ca(OH)2 in the char could absorb CO 2 released during the devolatilization,24 which caused the inhibition of the weight loss below 390 °C. Leftover Ca(OH)2 started to decompose at 400 °C,25 leading to the highest peak of the first stage at 450 °C. In the second stage, the peak temperature was shifted from 762 to 677 °C compared to char-260 and the maximum mass loss rate was raised from 0.057 to 0.092 mg/min. This was due to the catalysis by CaO on the steam gasification of remaining char.28 A shoulder peak was found at 750 °C, which was probably due to the decomposition of CaCO3 formed in the hydrothermal treatment and devolatilization stage. 3.2.2. Kinetic Analysis. The kinetic parameters of the heating process of dried raw sludge and hydrochars in a steam atmosphere were listed in Table 4. It could be found that the reactions in different regions could be accurately described by the diffusion model or second-order model. The activation energies of the hydrochars during devolatilization were much higher than that of the raw sludge, which was caused by the loss of highly active volatiles during the treatment. In the stage of steam gasification of remaining char, the activation energy was reduced from 231.0 kJ/mol for raw sludge to 209.8 kJ/mol for char-200 and 185.7 kJ/mol for char-260. This is evidence that the steam gasification activity of the sludge was raised with the hydrothermal extent. As mentioned above, the calcium inorganics formed during the hydrothermal treatment and devolatilization process have inhibition or promotion effects on weight loss in different temperature ranges, which led to the appearance of several new peaks. Hence, several sub-regions were divided according to these peaks. Therefore, kinetic parameters of char-260-CaO were not comparable to the other hydrochars. 3.3. Hydrothermal Treatment on the Emission of Gaseous N Compounds during Gasification. 3.3.1. Evolution of N Functionalities in Hydrochars with Hydrothermal Conditions. The relative contents of different N functionalities in dried sludge and hydrochars were displayed in Figure 4. Nitrogen in the raw sludge contained protein N (39.6%), inorganic N (17.7%), and heterocyclic N (42.7%), consisting of pyrrolic N (25.5%) and pyridinic N (17.2%). Heterocyclic N was from the decomposition of the products of nucleic acids, and its high content was in accordance with the findings of Tian et al.29 With hydrothermal treatment at 200 °C, inorganic NH4+/NO3− N in the raw sludge was totally dissolved in the aqueous phase; therefore, the corresponding peak was not identified in the spectrum. The protein N and pyrrolic N

260, which means that raising hydrothermal temperature favored the stabilization of FC in sludge. By the pre-addition of CaO, the ash content in the char at 260 °C was increased. However, the FC fraction was significantly reduced, from 7.16 to 1.23%. According to GBT212-2008, which was adopted for proximate analysis, the air-dried sample was heated in a close crucible at 900 ± 10 °C to determine the fraction of VM. The decomposition of Ca(OH)2 at 400−450 °C and CaCO3 at 825 °C25 created a steam and CO2 atmosphere. In addition, the solid product CaO played a role as a catalyst26 and promoted the gasification of FC with H2O and CO2. Therefore, a certain amount of FC migrated into the gaseous phase and was counted as VM. The oxygen fraction in the sludge was greatly reduced by hydrothermal treatment as a result of the decarboxylation or dehydration reaction.16 On the other hand, the hydrogen fraction was slightly reduced as a result of dehydration or demethanation.16 The treatment at 200 °C lead to 54.5% of nitrogen loss from the raw sludge. In addition, raising the temperature significantly favored the nitrogen removal, which was due to the solving of inorganic N and the conversion of organic N to ammonium N in sludge.27 Only 20.6% of nitrogen in raw sludge was retained in char-260. The removal rate of sulfur was higher than 35% by hydrothermal treatment, while the sulfur content in char was slightly increased. The addition of CaO before the treatment at 260 °C improved both the nitrogen and sulfur removal from the raw sludge. 3.2. Activity of Hydrochars during Steam Gasification. 3.2.1. TG Analysis. The differential thermogravimetric (DTG) curves of the steam gasification of dried raw sludge, char-200, char-260, and char-260-CaO at the heating rate of 8 K/min were displayed in Figure 3. According to the curves, the steam gasification of dried sludge and chars with no CaO added could

Figure 3. DTG curve of dried raw sludge and hydrochars in a steam atmosphere. D

DOI: 10.1021/acs.energyfuels.7b03304 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 4. Kinetic Parameters for Dried Raw Sludge and Hydrochars in Different Temperature Ranges sample dried sludge char-200 char-260 char-260-CaO

temperature range (°C) 210−522 578−810 210−542 557−819 210−542 555−848 210−250 270−340 356−528 544−725 741−801

f(α)

A (min−1)

E (kJ/mol)

r2

F2 D4 F2 D4 D4 D4 F2 F1 F1 D4 F2

× × × × × × × × × × ×

71.6 231.0 78.3 209.8 102.3 185.7 222.4 120.2 91.4 265.4 528.2

0.976 0.974 0.985 0.977 0.926 0.983 0.999 0.988 0.992 0.985 0.996

4.89 1.96 1.47 4.85 1.47 3.60 6.98 1.96 8.06 1.07 2.81

5

10 1010 106 108 106 107 1022 1010 105 1013 1026

Figure 4. Relative content of different nitrogen functionalities in dried sludge and hydrochars.

Figure 5. NH3/HCN emission from pyrolysis and steam gasification of dried raw sludge.

fractions were decreased by 3.8 and 15.9%, respectively. On the contrary, the proportion of pyridinic N was raised to 21.4%, which indicated that pyridinic N was relatively stable in this condition. When raising the treatment temperature to 260 °C, the protein fraction was reduced by 27.5%, while the heterocyclic N fraction was increased by 27.9%. Similarly, He et al.17 found that there was a pronounced decrease of protein N and increase of heterocyclic N in hydrochar of sludge when the temperature was raised to 260 °C. This could be explained by the polymerization of the proteins, resulting in the formation of the hetrocyclic N compounds from 200 to 260 °C.30 In char260-CaO, all of the organic N functionalities were less compared to char-260. This indicated that the cracking of them were promoted by CaO addition.17 3.3.2. Emission of N-Containing Compounds during Pyrolysis and Steam Gasification of Dried Raw Sludge. NH3 and HCN were identified as the N-containing compounds in the FTIR spectrum of the syngas from the gasification of the dried raw sludge and hydrochars. To reveal the impact of steam on the nitrogen discharge, the emission profiles of NH3/HCN during dried raw sludge pyrolysis and steam gasification were displayed in Figure 5. In pyrolysis, the emission of NH3 was from 150 to 700 °C. A total of 90.2% of the emission occurred below 500 °C as a result of the ammonium salt decomposition and protein deamination.30 On the other side, HCN emission started at 218 °C and stopped once it reached 850 °C, with the highest peak at 400 °C. By comparison of the emission profiles of HCN during pyrolysis of sewage sludge in this case and other studies,29,30 it could be concluded that the increase of

heterocyclic N in the sludge led to a shift of the peak temperature to the lower range. When steam was used as a gasification agent, both the NH3 and HCN emissions were significantly increased over the heating process. For NH3, the promotion was greater during the stage of steam gasification of remaining char. Tan et al.30 found that, in the pyrolysis of coal, the interaction of the H radicals from H2O with char N could produce NH3 above 700 °C. One reason for the promotion of HCN emission was the reforming of tar by steam, leading to the further cracking of nitrile N and heterocyclic N compounds. The hydrolysis of char N was another path as a result of the extension of HCN release time at a final temperature in a steam atmosphere. In comparison to pyrolysis, the conversion of sludge N to NH3 N was increased from 29.2 to 75.8%, while for HCN, it was increased from 7.6 to 22.8%. 3.3.3. Emission of N-Containing Compounds during Steam Gasification of Hydrochars. The emission profiles of NH3 and HCN during the steam gasification of the dried raw sludge and hydrochars were displayed in Figure 6. As in raw sludge gasification, two NH3 emission stages with the critical temperature around 500 °C were found in the profile of hydrochars. The release in the first stage was significantly reduced with the hydrothermal extent. For char-200 and char260, it was 42.3 and 18.9% of raw sludge. This was in accordance with the N functionality conversion of hydrochar with treatment conditions in XPS analysis, in which inorganic N was totally removed by the treatment and protein N was decreased with the hydrothermal extent. Ammonium and liable protein were the main precursors that contributed to NH3 E

DOI: 10.1021/acs.energyfuels.7b03304 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

therefore, the reduction at this range could be caused by the decrease of stable protein in the sample, as mentioned above. The pre-addition of CaO into the raw sludge disposed at 260 °C showed little effect on the total NH3 discharge, while the peak temperature of the second stage was apparently shifted to a lower temperature as in the DTG curve. CaO addition could promote the deamination of proteins and other N compounds in sewage sludge.24 In addition, the conversion of HCN to NH3 was also catalyzed.19 Therefore, the NH3 emission did not change, although the nitrogen content in the hydrochar was reduced by CaO addition. Meanwhile, the HCN emission was decreased by 26.9%.23

4. CONCLUSION The hydrothermal treatment on the steam gasification behavior of sewage sludge was studied, including the reactivity and gaseous N emission. The result showed that the activation energy of the remaining char gasification stage was decreased with the hydrothermal extent. In addition, the NH3 and HCN discharge was significantly reduced by the treatment as a result of the removal of inorganic N, the decomposition of protein N, and the change of the distribution of different heterocyclic N. The addition of CaO into raw sludge before the treatment led to the shift of the gasification temperature to the lower range and promoted the conversion of NH3 to HCN.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-21-65985009. Fax: +86-21-65982786. E-mail: [email protected]. ORCID

Yuheng Feng: 0000-0001-7614-3794 Dezhen Chen: 0000-0003-3421-3998 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 51706156), the Shanghai RisingStar Program (Grant 17QC1401000), the Chinese Academy of Sciences (CAS) Key Laboratory of Renewable Energy (Grant Y707k41001), and the Visiting Scholar Foundation of Key Laboratory of Clean Energy Utilization in Zhejiang University (Grant ZJUCEU2015017).

Figure 6. (a) NH3 and (b) HCN emissions from dried raw sludge and hydrochars in steam gasification.

emission below 500 °C.30 The NH3 emission in the second stage was also reduced, by 28.2% for char-200 and 40.8% for char-260. It could be found that the sequence of the peak temperature in this stage was corresponding to that in the DTG curve, which was raised with the hydrothermal extent. The emission of NH3 above 500 °C was from the ring opening of heterocyclic N in char and tar.30 However, there was no obvious decrease of heterocyclic N with the hydrothermal extent from XPS analysis. Hence, the NH3 reduction in the second stage might be mainly caused by the reduce of stable protein. This part of protein was not polymerized at a low temperature, while it started to decompose and contributed to the formation of heterocyclic N above 500 °C. The release of HCN was also decreased with the increase of the hydrothermal extent. The HCN production from char-260 gasification was 57.8% of the raw sludge. Because heterocyclic N in the sample was the primary source of HCN at a low temperature, the reduction below 500 °C was probably due to the variation of the distribution of pyrrolic N and pyridinic N. The ratio of pyrrolic N to pyridinic N was 1.48, 1.04, and 0.83 for dried raw sludge, char-200, and char-260, respectively. The cracking of heterocyclic N from polymerization of stable protein produced both NH3 and HCN above 500 °C;

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REFERENCES

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DOI: 10.1021/acs.energyfuels.7b03304 Energy Fuels XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.energyfuels.7b03304 Energy Fuels XXXX, XXX, XXX−XXX