Nitrogen Evolution during Fast Pyrolysis of Sewage Sludge under Inert

21 Jun 2017 - The influence of atmospheres on the product distribution and behaviors of nitrogen evolution during fast pyrolysis of sewage sludge (SS)...
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Nitrogen Evolution during Fast Pyrolysis of Sewage Sludge under Inert and Reductive Atmospheres Fu Wei, Jing-Pei Cao,* Xiao-Yan Zhao, Jie Ren, Jing-Xian Wang, Xing Fan, and Xian-Yong Wei Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), China University of Mining and Technology, Xuzhou, Jiangsu 221116, People’s Republic of China S Supporting Information *

ABSTRACT: The influence of atmospheres on the product distribution and behaviors of nitrogen evolution during fast pyrolysis of sewage sludge (SS) was investigated in a drop-tube quartz reactor. The results indicated that H2 improved the formation of gas products and gave a relatively low tar yield in comparison to an inert atmosphere. The char N yield obtained under a H2 atmosphere is lower than that under an Ar atmosphere. Above 500 °C, H2 further promoted the conversion of nitrogenous compounds to NH3. The HCN yield was low under all conditions. The decomposition of nitrogenous substances in SS produced more amine N, nitrile N, and heterocyclic N under a H2 atmosphere. The synergistic effect of a reductive atmosphere and high temperature promoted the thermal decomposition of more difficult-to-cleave N-containing heterocycles, such as piperidines, pyrroles, and pyridines. This study provides a better and deep understanding of the nitrogen transformations during fast pyrolysis of SS under a reductive atmosphere, which would benefit the environmental protection and sustainable clean use of SS. coal pyrolysis.18−20 It is clear that there are great differences between the nitrogenous substances in SS and coal, which may further lead to some certain differences in pyrolysis behaviors of nitrogen. An in-depth understanding of the mechanism of nitrogen removal under a reductive atmosphere may provide some theoretical support for the effect of H2 on nitrogen removal in the catalytic hydrodenitrogenation (HDN) research. It is of great significance in the pollution-free utilization of SS. Therefore, the reductive atmosphere is also an important factor that must be considered. Herein, we compared the effects of the reaction atmosphere and pyrolysis temperature on product distribution and carbon conversion, especially the transformation and evolution behaviors of nitrogen in gas, char, and tar during pyrolysis.

1. INTRODUCTION Currently, the harmless treatment and efficient utilization of sewage sludge (SS), the byproduct generated by effluent disposal,1,2 has gained extensive concern of the public. More attention has been paid to thermochemical conversion to solve the problem of the continuous increase in the SS amount and the resulting complex environmental problems.3 Among that, pyrolysis is being widely used because it is not only a vital intermediate stage in biomass conversion but also a promising, reliable, economic, and environmentally friendly disposal approach.4,5 Especially, fast pyrolysis has aroused the public special interest as a result of its ability to reduce waste deposition and eliminate toxic chemicals and pathogens while maximizing tar products for fuel and chemical use.6−8 Although SS pyrolysis has significant advantages, it is still difficult to overlook the high content of nitrogen-containing species (NCSs). During pyrolysis, the NCSs were converted to toxic nitrogenous gases, such as NH3 and HCN, which would be further transformed into NOx under certain conditions, causing serious photochemical fog and acid rain contamination.9−11 Accordingly, adequate knowledge of the conversion and distribution of NCSs during fast pyrolysis is vital for reducing emissions of harmful N-containing gases effectively. Nitrogen transformation during fast pyrolysis of SS has been studied under different reaction conditions, such as pretreatment method, residence time, reaction temperature, and heating rate.12−15 Most experiments were carried out under an inert atmosphere.12,16,17 Different from an inert atmosphere, the reductive atmosphere may significantly affect the conversion and distribution of nitrogen. Although the use of H2 as a reductive atmosphere is non-profitable, it is of great help to further study nitrogen removal during pyrolysis. To date, there are only a small amount of literature that studied the impact of a reductive atmosphere on nitrogen transformation in © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. SS Sample. The SS sample obtained from the sewerage management center in Xuzhou Guozhen, China, was pretreated as follows: (1) dried to a constant weight at 105 °C for 24 h in a drying oven and (2) milled and pulverized to be 0.4−1.0 mm particle size. Then, the treated sample was characterized by elementary analysis, thermogravimetric (TG) analysis, and Fourier transform infrared spectroscopy (FTIR), and the detailed preparation was presented in the Supporting Information. The proximate and ultimate analyses of SS are listed in Table 1. 2.2. Pyrolysis and Characterization. Fast pyrolysis of SS was carried out at a specified temperature (400−700 °C) in a drop-tube quartz reactor under a high-purity Ar or H2 atmosphere. The experimental device had been described previously.12 The pyrolysis products are mainly divided into char, tar, water, and noncondensable gases, and the tar yield was calculated by subtracting water from the liquid product. Tar can be divided into water-soluble oil (WSO) and Received: March 31, 2017 Revised: May 27, 2017 Published: June 21, 2017 A

DOI: 10.1021/acs.energyfuels.7b00920 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Proximate and Ultimate Analyses of SSa proximate analysis (wt %)

and H2 atmospheres, respectively. The trend was mainly attributed to further pyrolysis of SS and the pyrolysis volatile secondary cracking reaction (SCR) at high temperatures.22 H2 can promote SCR of the volatiles while preventing the polymerization of thermal decomposition products by inactivating the free radicals.23 Raising the temperature resulted in a slight increase in the moisture content of the liquid mixture, indicating that the high temperature can promote the gradual release of adsorbed water. A relatively high water content (ca. 10.0−15.0%) was obtained under a H2 atmosphere compared to an Ar atmosphere (ca. 9.8−13.2%), which is probably related to the conversion of oxygen in tar into water promoted under a H2 atmosphere.24 3.2. FTIR Analysis. FTIR spectra of SS and tar were exhibited in Figure 2. Similar nitrogen-containing functional

ultimate analysis (wt %, daf)

Mar

Ad

VMd

FCdb

C

H

N

S

Ob

H/C

6.2

65.4

32.5

2.1

47.4

7.7

8.3

2.4

34.2

1.9

M, moisture; A, ash; VM, volatile matter; FC, fixed carbon; ar, asreceived basis; d, dried basis; and daf, dry and ash-free basis. b Calculated by difference. a

water-insoluble oil (WISO) according to water solubility. The noncondensable gases collected by gas bag were determined using a Shimadzu GC-2014 gas chromatograph. The N-containing gases (NH3 and HCN) dissolved in deionized water (DIW) were quantified with a BANTE 931 ion meter. The total nitrogen content in WSO and the total organic carbon in DIW were determined with a Shimadzu TOCLCSN total organic carbon and total nitrogen analyzer. A KEM MKA710 Karl Fischer moisture titration was used to measure the water contents in the liquid product by the Karl Fischer titration method. Tars were analyzed using FTIR and gas chromatography/mass spectrometry (GC/MS). Quantification of the detected substances was performed using an external standard calibration curve constructed with five different concentrations. The detailed pyrolysis processes and instrument information are listed in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Product Yields. As depicted in Figure 1, with the increase of the temperature from 400 to 700 °C, the char yield

Figure 2. FTIR spectra of SS and tars.

Figure 1. Effects of the temperature and atmosphere on product yields.

groups were detected, indicating that SS and tar were both rich in NCSs. The interpretation of the FTIR spectra was mainly derived from previously literature.12,25−28 The amine/amide achieved the strongest absorption intensity at 400 °C, especially under a H2 atmosphere, which is mainly due to the breakage of peptide bonds in protein N to form more unstable and stable amine/amide compounds at a relatively low temperature.13 The reductive atmosphere may promote a more complete cleavage of protein N in SS. More H radicals produced during hydrocracking may inhibit polymerization of the amine N compounds.23 In addition, the reaction of H2 and nitrogenous compounds may also contribute to the production of amine/ amide compounds. With the further increase in the temperature, the intensity of the amine/amide peak gradually decreased, while the intensities of heterocyclic N and nitrile peaks were gradually becoming more prominent. The trend was particularly pronounced under a H2 atmosphere compared to an Ar atmosphere. It revealed that H2 has a promoting effect on

decreased significantly as a result of the gradual pyrolysis conversion of organic compounds in SS. Whether under an Ar or H2 atmosphere, the rate of declination in the range of 400− 600 °C was faster than that in 600−700 °C (40.0−23.1 and 36.0−14.2% versus 23.1−17.7 and 14.2−10.7%). The result indicated that the thermal cracking of SS was mainly concentrated in 400−600 °C.12 The yield of char under a H2 atmosphere was lower than that under an Ar atmosphere, indicating that H2 further contributed to thermal cracking of char, converting the stable oxygen-containing substances to gases by de/repolymerization.21 With an increasing temperature, the gas yield increased gradually and the total gas yield obtained under a H2 atmosphere was higher than that under an Ar atmosphere. The tar yield increased from 42.1 and 43.3% to 52.2 and 48.9% by raising the pyrolysis temperature from 400 to 500 °C and then sharply decreased to 48.6 and 43.7% by further raising the pyrolysis temperature to 700 °C under Ar B

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Energy & Fuels the cracking of labile NCSs at high temperatures, which produced more heterocyclic N and nitrile N compounds by the enhancement of char N (nitrogen in char) release.17 Meanwhile, the SCR of the amine/amide compounds as intermediates also contributed to the generation of heterocyclic N and nitrile N compounds through the polymerization reaction.13 3.3. Tar Analysis by GC/MS. The detected organic compounds can be classified into organooxygen species (OOSs), organonitrogen species (ONSs), aliphatics, and arenes.8 The detailed material is listed in Table S1 of the Supporting Information. Figure 3 showed that the ONSs

Figure 3. Effects of the temperature and atmosphere on the group component of tars.

Figure 4. ONS distributions in tars under different temperatures and atmospheres.

occupied fairly large proportions during the whole pyrolysis process under Ar and H2 atmospheres, which were inseparable from the presence of large amounts of NCSs in SS. The yield of ONSs increased rapidly with the increase of the pyrolysis temperature, especially under a H2 atmosphere. It indicated the ONSs derived from the decomposition of NCSs in SS, using H2 as the reaction gas, could further promote the conversion of the thermally stable NCSs into tar N (nitrogen in tar). The yield of arenes increased as the temperature increasing under both H2 and Ar atmospheres, which could be explained through the Diels−Alder reaction mechanism.29 The yield of aliphatics decreased at high temperatures between 500 and 700 °C as a result of the cracking of macromolecular aliphatics into lowmolecular-weight aliphatics. The OOSs decreased quickly with raising the pyrolysis temperature, especially under a H2 atmosphere between 400 and 500 °C. This reveals that most volatile OOSs are easily decomposed into hydrocarbons from 400 to 500 °C and more readily converted to gaseous products rather than liquid species at a high temperature through hydrocracking and hydrodeoxygenation.30 The ONSs mainly include amine N, nitrile N, and heterocyclic N.31,32 As shown in Figure 4, the amine N yield under an Ar atmosphere decreased from 56.4 to 32.1 mg/g below 600 °C, while the amine N yield decreased from 52.3 to 28.3 mg/g under a H2 atmosphere. With further raising the temperature to 700 °C, the amine N yield decreased slowly under an Ar atmosphere, while the trend under a H2 atmosphere was less obvious. This is mainly because hydrocracking may provide more H radicals, which can inhibit polymerization of free radical fragments and other unstable molecules into macromolecular compounds at low temperatures. This led to the cleavage of peptide bonds in proteins to

produce more small-molecule amine N compounds at low temperatures.17 Hydrocracking may promote the secondary thermal cracking of stable amine N with the increase of the temperature.33 With the temperature increasing from 400 to 700 °C, heterocyclic N increased more rapidly from 79.5 to 164.5 mg/g and from 81.9 to 191.1 mg/g under Ar and H2 atmospheres, respectively. The rate of heterocyclic N increase under a H2 atmosphere was faster than that under an Ar atmosphere. The results show that hydrocracking could promote the thermal cracking of stable ONSs to produce more heterocyclic N compounds.17 The SCR of intermediate amine N may further generate gas N (nitrogen in gas), heterocyclic N, and nitrile N, which is consistent with pyrolysis of the amino acids.34 Aromatic nitrile N increased continuously under Ar and H2 atmospheres. The aliphatic nitrile N increased from 5.3 to 31.5 mg/g between 400 and 600 °C under an Ar atmosphere and then decreased to 22 mg/g when the temperature further increased to 700 °C. The decomposition temperature of aliphatic nitrile N decreased under a H2 atmosphere, and aliphatic nitrile N began to decompose at 500 °C. It indicated that aromatic nitrile N had better thermal stability than aliphatic nitrile N. Aliphatic nitrile N was susceptible to thermal decomposition in HCN at a high temperature by hydrocracking. A similar finding reported that SCR of nitrile N favors the release of HCN.13,35 During the complex pyrolysis reactions, heterocyclic N, including pyridines, pyrroles, indoles, quinolines, and piperidines, are the most abundant in ONSs, which may be generated by inherent heterocyclic N conversion in SS and the polymerization of amine N.17,36 With the temperature increasing from 400 to 700 °C, indoles, pyridines, and quinolines showed increasing trends gradually with the C

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Ar atmosphere. This indicates that the decomposition of proteins, the main components of SS, occurred mainly in the range of 400−500 °C, which was well-consistent with the TG analysis in the Supporting Information.12 Unlike under an Ar atmosphere, the yields of char C and char N above 500 °C decreased faster under a H2 atmosphere, especially the char N yield. The char C yield reduced from 32.4 to 28.5% and reached the minimum value of 25.4%, while the char N yield decreased rapidly from 27.9 to 12.1%. The results indicated that most relatively labile organic carbon released completely around 500 °C under an Ar atmosphere and hydrocracking further promoted cracking of stable organic carbon in char at a high temperature. The yield of char N decreased faster than that of char C, indicating that hydrocracking is more likely to promote the thermal decomposition of stable NCSs compared to cracking of stable organic carbon. This is mainly because more H radicals were produced by the hydrogenation reaction and adsorbed on the char surface, which further facilitated the conversion of thermally stable NCSs to gas N and tar N by coaction with thermal cracking.40,42 The maximum conversion rates of SS C to tar C (carbon in tar) at 500 °C were 57.2 and 57.6% under Ar and H2 atmospheres, respectively. The yield of WSO C (carbon in WSO) was lower than that of WISO C (carbon in WISO) under Ar and H2 atmospheres, while the WSO N (nitrogen in WSO) yield was much higher than the WISO N (nitrogen in WISO) yield. It showed that most ONSs in tar were WSO. Above 500 °C, raising the temperature decreased the yields of WISO C rapidly because of the SCR of volatile matters.21 The trend was more obvious under a H2 atmosphere, leading to a sharp increase in the gas C (carbon in gas) yield. The yields of WSO C under a H2 atmosphere were generally higher than that under an Ar atmosphere. This demonstrates that secondary hydrocracking can improve the quality of tar while producing more light hydrocarbons.23 The WSO N yield increased with an increasing pyrolysis temperature under Ar and H 2 atmospheres, and a relatively high yield was obtained under a H2 atmosphere. It can be speculated that hydrocracking may promote the decomposition of WISO N into WSO N. The major gaseous nitrogen products detected in the SS pyrolysis process were NH3 and HCN. The NH3 yield under an Ar atmosphere was 13.8% at 400 °C, which was lower than that of 14.9% under a H2 atmosphere. The HCN yield only accounted for about 1.6% under both Ar and H2 atmospheres. This is mainly because of thermal decomposition of inorganic nitrogen compounds and labile NCSs, and the hydrocracking reaction may bring the thermal decomposition of stable NCSs forward.17 As the pyrolysis temperature was raised to 500 °C, the NH3 yield rapidly increased to 21.0 and 26.3% under Ar and H2 atmospheres, respectively. The HCN yield only increased by 0.7% under a H2 atmosphere. The decomposition of abundant proteins in SS and deamination of amine N in tar in the stage of 400−500 °C are conducive to the formation of large amounts of NH3.12,17 The synergistic effect of the H radical and thermal cracking further promoted the decomposition of more stable NCSs to produce NH3 and HCN.42,43 With an increasing temperature from 500 to 700 °C, the NH3 yield increased to 29.0 and 43.4%, with the yield of HCN increasing to 2.6 and 3.1%, under Ar and H2 atmospheres, respectively. The SCR of intermediates (stable amine N, nitrile N, and heterocyclic N) may be responsible for the generation of NH3 and HCN.18,20 The H2-assisted SCR during pyrolysis of SS will be propitious to the formation of sufficient H radicals,

continuous decrease of piperidines under Ar and H 2 atmospheres. This suggested that indoles, pyridines, and quinolines are the main heterocyclic N products in the thermal decomposition of SS N. The thermal stability of piperidines is poorer than those of indoles, pyridines, and quinolines. The pyrrole yield decreased obviously from 23.1 to 12.7 mg/g under an Ar atmosphere with the temperature raising from 500 to 600 °C, indicating that the thermal decomposition of pyrroles may occur in this temperature range.37,38 Other heterocyclic N had relatively high contents between 600 and 700 °C under a H2 atmosphere compared to that under an Ar atmosphere, except for pyridines and piperidines. The results showed that the hydrocracking reaction further promoted the cracking of NCSs in SS to produce more heterocyclic N. More H radicals released during the hydrocracking of SS, which is favorable for the ring opening of heterocyclic N.39 The SCR of heterocyclic N, such as piperidines, pyrroles, and pyridines, by the hydrogenation reaction may be conducive to the release of HCN between 600 and 700 °C.17,36,40 3.4. Carbon and Nitrogen Balance Analyses. As illustrated in panels a and b of Figure 5, the release of SS C

Figure 5. Effects of the temperature and atmosphere on (a) carbon and (b) nitrogen distribution.

(carbon in SS) and SS N strongly depended upon the temperature and atmosphere and showed different distribution regularity. The yield of char C (carbon in char) was higher than that of char N under Ar and H2 atmospheres. This reveals that the thermal decomposition of SS N was easier than that of SS C.41 In comparison to the rapid decrease in the char C and char N yields between 400 and 500 °C, the rate of declination in char C and char N yields was quite slow above 500 °C under an D

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3.5. Nitrogen Transformation. On the basis of a comprehensive analysis of the above conclusions, the possible reaction routes of nitrogen conversion during the fast pyrolysis of SS were roughly presented in Figure 7. Synergies of the temperature and atmosphere have a significant effect on the release and transformation of SS N during the pyrolysis process. Below 400 °C, the direct cleavage of inorganic nitrogen compounds and labile proteins could be responsible for the release of NH3. The deamination and dehydrogenation of labile amine N produced by thermal decomposition of SS N also contributed to the formation of NH3 and HCN. The generation of HCN below 400 °C is mainly attributed to the thermal decomposition of inherent heterocyclic N in SS. The reaction temperature plays a major role in the release of NH3 compared to the reductive atmosphere in this temperature range. H2 can provide abundant H radicals during SS pyrolysis. The hydrogenation reaction of the SS N surface by the H radical can promote the decomposition of stable proteins to produce amine N, heterocyclic N, and nitrile N. With the increase of the temperature, the dehydrogenation and polymerization of amine N are in favor of the generation of heterocyclic N and nitrile N. SCR of nitrile N can lead to the release of HCN, which could be further converted to NH3 by the reaction with H2. Hydrogenation makes the ring-opening effect of heterocyclic N, including pyrroles, pyridines, and piperidines, more prominent in the range between 600 and 700 °C. The H radical may attack the N site of heterocyclic N, thereby promoting the formation of NH3. As shown in Table 1, the SS has a certain content of sulfur. The sulfur-containing species (SCSs) derived from pyrolysis and/or hydrogenation pyrolysis of coal and SS are mainly in the form of H2S.46,47 The SCSs in the pyrolysis volatiles are usually more easily removed than NCSs in the presence of H2, which means that H2S produced by hydrodesulfurization may have an influence on HDN reactions. The influence of SCSs on nitrogen evolution will be carried out in our future work.

which contribute to hydrogenation of the N site of heterocyclic N to form NH3.17 The selectivity of nitrogen transformation under different conditions can be seen more intuitively by the HCN/NH3 ratio. As shown in Figure 6, the temperature and atmosphere

Figure 6. Molar ratio of HCN and NH3 under different temperatures and atmospheres.

had a very significant impact on the HCN/NH3 ratio. At 400 °C, the HCN/NH3 ratio under a H2 atmosphere was lower than that under an Ar atmosphere, indicating that hydrocracking may facilitate the thermal decomposition of stable amine N to produce more NH3 at a low temperature.17 The HCN/NH3 ratio under a H2 atmosphere decreased rapidly with an increasing temperature, while the HCN/NH3 ratio decreased in the range of 400−600 °C and then gradually increased when the temperature increased to 700 °C under an Ar atmosphere. The HCN/NH3 ratio under an Ar atmosphere was lower than that under a H2 atmosphere between 500 and 600 °C, indicating that hydrocracking of char N promoted the production of more HCN at this stage. The HCN/NH3 ratio under an Ar atmosphere was higher than that under a H2 atmosphere at 700 °C. The results shown that hydrocracking is conducive to deeper cleavage of stable nitrile N and heterocyclic N in char N and tar N to produce more NH3 and less HCN.20,39 Besides, the increment of the NH3 yield is higher than the decrement of the char N yield, as shown in Figure 5, showing that H2 is favorable to convert HCN into NH3 at a high temperature.41,44,45

4. CONCLUSION The distribution of pyrolysis products and the transformation behaviors of carbon and nitrogen showed significant differences in SS pyrolysis under inert (Ar) and reductive (H2) atmospheres. In comparison to an Ar atmosphere, more stable organic matter decomposed to gases under a H2 atmosphere.

Figure 7. Possible reaction routes of nitrogen conversion. E

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Besides, the hydrocracking reaction promoted the conversion of WISO C into gas C, further enhancing the quality of tar. The assistance of H2 facilitated the thermal decomposition of stable NCSs in SS into amine N, heterocyclic N, and nitrile N. Between 400 and 500 °C, the SCR of amine N was conducive to the formation of NH3, heterocyclic N, and nitrile N. With the pyrolysis temperature increased to 700 °C, the reductive atmosphere not only contributed to the SCR of heterocyclic N and nitrile N to produce more NH3 and HCN but also had a promotive function in the conversion of HCN to NH3.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b00920. Pyrolysis processes, device operation information (BANTE 931 ion meter, elementary analysis, TG, FTIR, and GC/MS), and instrument information shown in the pyrolysis and characterization section, TG analysis of SS shown in the thermal decomposition behavior of SS section and details of substances detected in tar by GC/MS listed in Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-516-83591059. E-mail: caojingpei@ cumt.edu.cn, [email protected], and/or beyondcao_ [email protected]. ORCID

Jing-Pei Cao: 0000-0002-1544-7441 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was subsidized by the Fundamental Research Funds for the Central Universities (China University of Mining and Technology, 2015XKQY05), the National Natural Science Foundation of China (Grant 21676292), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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DOI: 10.1021/acs.energyfuels.7b00920 Energy Fuels XXXX, XXX, XXX−XXX