Hydrocarbon and Ammonia Production from Catalytic Pyrolysis of

Feb 8, 2016 - ... atmosphere and the solid residue was removed from the sample cup for analysis. ..... 2014, 155, 289– 299 DOI: 10.1016/j.biortech.2...
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Research Article pubs.acs.org/journal/ascecg

Hydrocarbon and Ammonia Production from Catalytic Pyrolysis of Sewage Sludge with Acid Pretreatment Guangyi Liu,†,‡ Mark Mba Wright,*,§ Qingliang Zhao,*,†,∥ and Robert C. Brown‡,§ †

School of Municipal & Environmental Engineering and ∥State Key Laboratory of Urban Water Resources and Environments (SKLUWRE), School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China ‡ Bioeconomy Institute, Iowa State University, 1140 Biorenewables Research Laboratory, 617 Bissell Road, Ames, Iowa 50011, United States § Department of Mechanical Engineering, Iowa State University, Black 2078, Ames, Iowa 50011, United States ABSTRACT: Sewage sludge is the major byproduct generated in wastewater treatment plants and a potential resource for biofuel production. This study investigated the catalytic pyrolysis (CP) of sewage sludge with HZSM-5 (CBV2314) in a microfurnace pyrolyzer for the production of hydrocarbons and ammonia. Pyrolysis temperature had a significant role in the production and selectivity of aromatic hydrocarbons, olefins, and alkanes. Acid pretreatments including infusion and washing were employed to alleviate the catalytic effects of ash content in sewage sludge and improve the hydrocarbon production of CP. Both acid-infusion and acid-washing enhanced hydrocarbon production but acid-infusion yielded higher hydrocarbon production than acid-washing. At 650 °C the optimal acid infusion loading was 1 mmol·g−1 sewage sludge and hydrocarbon yields reached to 51.4%. CP of acid-pretreated sewage sludge enhanced the denitrogenation of volatiles by releasing nitrogen as ammonia which could be recycled as fertilizer. These findings demonstrated that CP with acid pretreatment is a promising approach to converting sewage sludge into valuable fuels and chemicals. KEYWORDS: Sewage sludge, HZSM-5, Catalytic pyrolysis, Acid pretreatment, Hydrocarbon, Nitrogen

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INTRODUCTION Sewage sludge is the major byproduct generated in wastewater treatment plants (WWTP).1 Global population growth and industrial activity has increased the use of wastewater purification systems and thus the production of sewage sludge. As a potential source of heavy metal and pathogen containing wastes, sewage sludge is becoming one of the most significant challenges in wastewater management.2 The most common disposal processes for sewage sludge are landfill, agricultural application, and incineration.3 These traditional disposal routes face environmental and social concerns. In landfills, gases generated from sludge organic matter are rich in methane, which is a greenhouse gas with 25 times the climate forcing impact than CO2.4 Furthermore, water runoff from landfills form leachate containing solubles and solids that need complex disposal treatments.5 Sludge can alternatively be used as a fertilizer because it contains organic matter, nitrogen, and phosphorus, which are nutrients for soils.6 However, heavy metals, pathogens, and some persistent organic contaminants (PAHs, PCBs, dioxins, and furans) in sludge could negatively affect the environment and human health.7 Finally, although sludge incineration can reduce waste volume by 70% and thermally destruct pathogens and toxic organic compounds,8 it faces major challenges such as toxic fly ash, high cost, and negative public perception.9 © 2016 American Chemical Society

Pyrolysis is an increasingly attractive process for sewage sludge management because recent developments allow it to achieve up to 50% reduction of the waste volume,10 stabilize sludge organic matter and metals,11 and produce valuable fuels and chemical products from the pyrolysis liquids or bio-oils. However, bio-oil from sludge pyrolysis has a relatively high nitrogen and oxygen content which limits its use as fuel.12 Catalysts can remove nitrogen and oxygen from bio-oil compounds. Zeolite catalysts for deoxygenation and denitrogenation have been successfully used in catalytic pyrolysis (CP) of lignocellulosic biomass,13,14 proteinaceous feedstocks,15,16 and municipal solid waste.17,18 However, articles on fast pyrolysis of sewage sludge with zeolite catalyst are limited. Kim and Parker19 investigated the effect of zeolite and acid/base pretreatment on the product distributions from sewage sludge pyrolysis and claimed none of these pretreatments improved the bio-oil yield. However, the zeolite consisted of pure SiO2 without a catalytically active metal and information on the composition of the bio-oil was not provided. Xie et al.20 conducted microwave-assisted catalytic pyrolysis of sewage sludge with ZSM-5 and found that 550 °C was the optimal Received: January 4, 2016 Revised: February 5, 2016 Published: February 8, 2016 1819

DOI: 10.1021/acssuschemeng.6b00016 ACS Sustainable Chem. Eng. 2016, 4, 1819−1826

Research Article

ACS Sustainable Chemistry & Engineering Table 1. Characteristics of Sewage Sludge Samplesa sewage sludge

original

acid-infusedb

acid-washed

acid-infusedc

acid-infusedd

e

moisture volatile fixed carbonf ash

5.6 ± 0.3 58.5 ± 0.3 3.9 ± 1.2 32 ± 1.2

carbon hydrogen nitrogen oxygenf C/N ratio (wt %)

35.1 4±0 4.3 24.6 8.2

proteinh lipid lignin carbohydratef SiO2 CaO K2O MgO Na2O Fe2O3 P2O5 Al2O3 MnO ZnO CuO SO3

± 0.1 ±0 ± 1.1 ±0

43.1 7.9 24.4 24.6

± ± ± ±

1 1.2 1.4 3.6

6.4 7.6 0.4 0.6 0.3 7.5 4.8 1.7 0.2 0.4 0.2 2.8

± ± ± ± ± ± ± ± ± ± ± ±

0.3 0.4 0.1 0 0 0.2 0.6 0.1 0 0.1 0 0.1

Proximate Analysis (wt %) 5.8 ± 0.6 5.5 ± 69.7 ± 0.7 56.4 ± 4.3 ± 1.3 3.7 ± 20.2 ± 1.4 34.4 ± Ultimate Analysise (wt %) 44.2 ± 0.4 32.1 ± 4.4 ± 0.2 4.1 ± 4.5 ± 0.1 4.1 ± 26.7 ± 0.7 25.3 ± 9.8 ± 0.1 7.9 ± Biochemical Compositiong (wt %) 38 ± 0.2 42.6 ± 7.4 ± 0.2 7.5 ± 28.5 ± 1.8 24.5 ± 26.1 ± 1.8 25.4 ± Mineral Compositione (wt %) 8.3 ± 0.2 6.2 ± 0±0 7.3 ± 0±0 0.4 ± 0±0 0.7 ± 0±0 0.3 ± 4.5 ± 0.4 7.1 ± 1.7 ± 0.3 4.3 ± 0.9 ± 0.2 1.5 ± 0±0 0.2 ± 0.1 ± 0 0.4 ± 0±0 0.2 ± 3.7 ± 0.2 6.5 ±

0.6 0.6 0.3 0.3

5.4 53.8 2.6 38.2

± ± ± ±

0.4 0.6 0.4 0.6

5.6 49.3 2.6 42.5

± ± ± ±

0.1 0.2 0.5 0.8

0.4 0.3 0.2 0.8 0.5

31.8 3.7 3.8 22.5 8.4

± ± ± ± ±

0.1 0.2 0.1 0.8 0.2

28.2 ± 3.7 ± 3.6 ± 22 ± 0.6 7.8 ±

0.2 0.1 0.1

2.3 0.6 1 2.6 0.4 0.3 0.1 0.1 0.1 0.5 0.3 0.2 0 0.1 0 0.1

42.1 ± 1 8 ± 0.9 25 ± 1.5 24.9 ± 1.6 6.1 ± 7 ± 0.4 0.3 ± 0.5 ± 0.3 ± 6.6 ± 4.2 ± 1.6 ± 0.2 ± 0.4 ± 0.2 ± 10.2 ±

0.4 0 0.1 0 0.3 0.5 0.2 0 0 0 0.1

0.2

43.3 ± 0.6 8.1 ± 0.7 23.5 ± 2.1 25 ± 0.8 5.3 6.1 0.3 0.5 0.2 6.2 3.9 1.3 0.2 0.4 0.2 16.5

± ± ± ± ± ± ± ± ± ± ± ±

0.5 0.3 0.1 0 0 0.6 0.4 0.2 0 0.1 0.1 0.2

“± number” refers to standard deviation. b0.5 mmol sulfuric acid/g (sewage sludge). c1.0 mmol sulfuric acid/g (sewage sludge). d2.0 mmol sulfuric acid/g (sewage sludge). eWet basis. fCalculated by difference. gDry and ash-free basis. hCalculated as 6.25 × nitrogen.

a

pretreatment, sewage sludge samples were either washed28 or infused24 by sulfuric acid (Fisher Scientific, USA) diluted to 0.1 mol·L−1. For acid washing, sludge sample was immersed and stirred by a magnetic stirrer with sufficient sulfuric acid solution for 12 h at ambient temperature and then rinsed with deionized water until the pH value was neutral. For acid-infusion, sewage sludge samples were immersed and stirred by a magnetic stirrer with sulfuric acid (Fisher Scientific, USA), diluted to 0.1 mol·L−1, at three different loadings (5, 10, 20 mL· g−1 sewage sludge) at ambient temperature. Finally, both acid washed sludge sample and acid infused sludge sample were dried in an oven at 105 °C for 24 h, and then stored in sealed glass vials in a desiccator before use. Ultimate analysis of sludge was performed using a Vario MICRO cube carbon/hydrogen/nitrogen elemental analyzer (Elementar, Germany). Proximate analysis was performed using a thermogravimetric analysis (TGA) system (Mettler Toledo, USA) following ASTM D5142. Lipid content was determined by Soxhlet extraction method.29 Lignin content was determined according to ASTM D110696 and the effect of inorganic compounds such as SiO2 was eliminated since the lignin content was calculated as the weight loss after igniting in muffle furnace.30 Protein content was determined according to AOAC International method.31 The carbohydrate content was calculated by difference.32 Ash content test was performed using the X-ray florescence (XRF) technique.22 The characteristics of sewage sludge samples are listed in Table 1. All analysis results are based on at least duplicate tests to check for reproducibility. As shown in Table 1, sewage sludge sample contains 5.6 wt % water and 32 wt % ash. It also contains 35.1 wt % carbon. Protein is the primary constituent at 43.1 wt % (daf: dry and ash-free basis) with equal portions of lignin and carbohydrates of about 24 wt % (daf). The primary mineral identified was CaO at 7.6 wt %. Acid-infusion pretreatment showed negligible

temperature for bio-oil production; however, the study did not report the composition of nitrogen-containing gas emissions. Inorganic compounds in biomass influence the thermal decomposition by promoting the yield of char and low molecular weight oxygenates in the pyrolysis process.21,22 Acid pretreatments have been developed to mitigate the catalytic activity of inorganic compounds by either washing out the metals23 or passivating the metals from interacting with the organic components.24 Sewage sludge contains high concentrations of inorganic compounds which would influence the pyrolysis process. Acid pretreatment of sewage sludge in pyrolysis has been reported to improve the quality of bio-oil25 and enhance the adsorptive property of biochar.26 However, research on the effect of acid pretreatment on catalytic pyrolysis of sewage sludge is limited. In this study, we conducted catalytic pyrolysis of sewage sludge with HZSM-5 in a microfurnace pyrolyzer to explore the potential of sewage sludge for production of hydrocarbons and ammonia. In addition, sewage sludge, acid-washed sewage sludge, and acid-infused sewage sludge were employed to investigate the effect of acid pretreatment on the fate of carbon and nitrogen in the catalytic pyrolysis of sewage sludge.

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MATERIALS AND METHODS

Materials. Digested sewage sludge was fetched from the Water Pollution Control plant located in Ames, Iowa, USA. The samples were dried at 105 °C for 24 h, then cooled in airtight containers, ground and sifted to olefins > alkanes. Yields for these

Figure 1. Schematic diagram of the microfurnace pyrolyzer. within the range of 40 to 900 °C with an interface heater operating at temperatures of 100 to 400 °C that prevents undesired temperature drops of the pyrolysis vapors as they exit the furnace. For a typical run, approximately 5 mg of catalyst/sludge mixture was pyrolyzed in the first furnace at the desired temperature, and the temperature of the second furnace and the interface were held at 320 °C to prevent product condensation. The products were analyzed by a gas chromatograph (GC, 7890A, Agilent Technologies, USA) installed with a three-way splitter that directed the gas stream to three GC columns. The pyrolyzer and GC with detectors were connected together so the pyrolysis process and analysis process were conducted at the same time. The GC oven temperature was programmed for a 3 min hold at 40 °C then increased at a 10 °C·min−1 rate to 250 °C and held constant for 6 min. The injector temperature was 250 °C and the total helium flow passing through the reactor was 90 mL·min−1. Two identical capillary columns, Phenomenex ZB 1701 (60 m, 0.250 mm, and 0.250 μm film thickness) were used to analyze the condensable volatiles: one was connected to a mass spectrometer (MS) (5975C,

Figure 2. Carbon distributions in catalytic pyrolysis of sewage sludge at different pyrolysis temperatures with catalyst to biomass ratio of 20:1. 1821

DOI: 10.1021/acssuschemeng.6b00016 ACS Sustainable Chem. Eng. 2016, 4, 1819−1826

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ACS Sustainable Chemistry & Engineering

selectivity was relatively stable indicating that pyrolysis temperature had greater effect on monocyclic aromatic hydrocarbon selectivity than polycyclic aromatic hydrocarbon selectivity during catalytic pyrolysis of sewage sludge. As shown in Figure 4, olefins produced during catalytic pyrolysis of sewage sludge were categorized as ethylene,

three hydrocarbons increased along with increasing pyrolysis temperature. Carbon yield of aromatic hydrocarbons increased from 17.7% at 450 °C to 24.8% at 750 °C. Carbon yield of olefins and alkanes were 8.5% and 3.3% at 450 °C respectively and increased to 13.5% and 11.8% at 750 °C, respectively. The carbon balance at 450 °C was worse than that at higher temperatures. This is because that the zeolite catalyst was less reactive at low temperature and part of the volatiles were not completely converted into hydrocarbons resulting the relatively worse carbon balance at 450 °C. This phenomenon was also observed by Wang.34 The increasing hydrocarbons yields under higher pyrolysis temperatures could be because volatiles released under higher reaction temperatures are smaller and more likely to travel through catalyst pores for deoxygenation.35 Additionally, diffusion rates under higher pyrolysis temperatures are enhanced and facilitate hydrocarbon formation.36 CO and CO2 showed the same trend in the temperature range with an increasing carbon yield from 5.4% and 5.3% at 450 °C respectively to 11.2% and 10.4% at 750 °C, respectively. Compared to hydrocarbons production of 45.3% obtained in catalytic pyrolysis of sewage sludge at 650 °C, hydrocarbons production was only 10.4% in noncatalytic pyrolysis. Meanwhile, the yields of CO and CO2 were 2.8% and 7.6%, respectively for noncatalytic pyrolysis, while 8.5% and 8.4% of CO and CO2 were obtained in catalytic pyrolysis, respectively. These results indicated that in catalytic pyrolysis of sewage sludge, zeolite catalyst promoted deoxygenation process and enhanced the production of hydrocarbons. Aromatic hydrocarbons were categorized as benzene, toluene, xylene, C9, and C10+ compounds. Their selectivities as a function of pyrolysis temperature are shown in Figure 3.

Figure 4. Carbon selectivity of olefins in catalytic pyrolysis of sewage sludge at different pyrolysis temperatures with catalyst to biomass ratio of 20:1.

propene and butene. Among olefin products, ethylene and propene prevailed over butene in the pyrolysis temperatures tested. Increasing pyrolysis temperature had little effect on selectivity of butene while the selectivity of ethylene increased from 34.1% at 450 °C to 47.9% at 750 °C and the selectivity of propene decreased from 47.5% at 450 °C to 37.3% at 750 °C. The reverse trends of selectivity of ethylene and propene are consistent with the general observation that increasing reaction temperature enhanced cracking reactions to produce small olefins.39 Similar to olefins, alkanes were categorized as methane, ethane, propane and butane in Figure 5. Pyrolysis temperature had significant effect on selectivity of alkane in catalytic pyrolysis of sewage sludge. At 450 °C propane and butane were

Figure 3. Carbon selectivity of aromatic hydrocarbons in catalytic pyrolysis of sewage sludge at different pyrolysis temperatures with catalyst to biomass ratio of 20:1.

Between 450 and 750 °C benzene selectivity increased considerably from 14.7% to 23.3% while xylenes and C9 decreased from 29.4% and 10.4% at 450 °C respectively to 22.9% and 7.2% at 750 °C, which can be attributed to enhanced dealkylation reactions at higher temperatures.37 As the major aromatic hydrocarbon, toluene selectivity was relatively stable from 450 to 750 °C. This can be explained by the balance between conversion of toluene into benzene through dealkylation and the formation of toluene from xylenes and C9 through dealkylation.37,38 From 450 to 750 °C, C10+ aromatics

Figure 5. Carbon selectivity of alkanes in catalytic pyrolysis of sewage sludge at different pyrolysis temperatures with catalyst to biomass ratio of 20:1. 1822

DOI: 10.1021/acssuschemeng.6b00016 ACS Sustainable Chem. Eng. 2016, 4, 1819−1826

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ACS Sustainable Chemistry & Engineering

yields. Addition of sulfuric acid at 2 mmol·g−1 (sewage sludge) showed similar carbon distribution to that of 1 mmol·g−1 (sewage sludge) which indicates that 1 mmol·g−1 (sewage sludge) is the optimal sulfuric acid loading for hydrocarbon production in catalytic pyrolysis of sewage sludge. Similar to acid-infused sewage sludge, acid-washed sewage sludge also obtained higher yields of hydrocarbon products and lower yields of carbonaceous residue. However, less improvement on hydrocarbons production was obtained in catalytic pyrolysis of acid-washed sewage sludge (aromatic hydrocarbons, olefins, and alkanes of 25.3%, 13.4%, and 9.1%, respectively). Accordingly, as showed in Table 2, productions of each specific aromatic hydrocarbon, olefin and alkane were also less enhanced after acid-washing pretreatment. This result is different from that of Wang et al.42 who reported that both acid-washing and acid-infusion improved catalytic pyrolysis of red oak and showed almost identical hydrocarbons production. This could be explained by the different compositions of red oak and sewage sludge. Red oak is mainly comprised of cellulose, hemicellulose and lignin which are water-insoluble so the acid-washing pretreatment mainly washed off the inorganic salts and improved the performance of catalytic pyrolysis. However, sewage sludge contains some oligosaccharides, amino acids and other organic compounds which are water-soluble, so the acid-washing pretreatment could not only wash off the ash content but also the soluble organic compounds resulting in relatively smaller improvement. Nitrogen Distributions in Pyrolysis of Sewage Sludge. Compared to lignocellulosic biomass, sludge has relatively high nitrogen content. This paper conducted noncatalytic and catalytic pyrolysis of sewage sludge, acid-washed sewage sludge and acid-infused (1 mmol·g−1) sewage sludge at 650 °C to investigate their effects on nitrogen distribution. As shown in Figure 7, acid pretreatment decreased nitrogen content in char and increased both ammonia and hydrogen cyanide yields during noncatalytic pyrolysis of sewage sludge. In catalytic pyrolysis, large amounts of nitrogen were released as ammonia for all three kinds of sewage sludge which is consistent to findings in previous studies.43,44 Among the three kinds of sewage sludge, acid-infused sewage sludge gave the greatest yield of ammonia of 61.3% probably due to the increase in volatiles released from the residue. This large amount of ammonia formed through denitrogenation of volatiles could be recycled as fertilizer to increase its economic feasibility. However, for all three kinds of sewage sludge, hydrogen cyanide yields were around 13% which would require proper removal measures.

the main components of alkanes while methane and ethane were undetectable. However, the selectivity of methane drastically increased to 63.6% at 750 °C indicating that high temperature favored production of small alkanes in catalytic pyrolysis of sewage sludge. Figure 6 shows the nitrogen distribution in the products of catalytic pyrolysis of sewage sludge as a function of pyrolysis

Figure 6. Nitrogen distributions in catalytic pyrolysis of sewage sludge at different pyrolysis temperatures with catalyst to biomass ratio of 20:1.

temperature. At 450 °C, 30.6% of the nitrogen was released as ammonia, while 49.7% was found in the solid residue of catalytic pyrolysis. As reaction temperature increased, nitrogen in the solid residue decreased and only 18.6% was observed at 750 °C. Accordingly, 63.4% of nitrogen was released as ammonia at 750 °C. This suggests there is potential for recycling ammonia as fertilizer for the agriculture industry. At 750 °C, however, 13.2% of nitrogen was released as HCN which is toxic and will require proper removal measures. Catalytic Pyrolysis of Acid-Pretreated Sewage Sludge. Previous research on the effect of inorganic salts on pyrolysis indicated significant effects on the thermal decomposition process.40,41 Since sludge has relatively high ash content, this paper compared catalytic pyrolysis of sewage sludge, acidinfused sewage sludge and acid-washed sewage sludge at 650 °C. Products distributions and selectivity are compared in Table 2. As shown in Table 2, the infusion of sulfuric acid at 0.5 mmol·g−1 (sewage sludge) had negligible effect on the carbon distribution of catalytic pyrolysis of sewage sludge. However, when the acid addition was 1 mmol·g−1 (sewage sludge) the carbon yield of aromatic hydrocarbons, olefins and alkanes increased to 26.6%, 14.1% and 10.8%, respectively. Accordingly, the carbonaceous residue decreased from 33% to 22.1%. Patwardhan et al.22 conducted fast pyrolysis of cellulose with different inorganic salts and found that the impregnation of inorganic salts favored the formation of char and low molecular weight species. Kuzhiyil et al.24 alleviated the influence of inorganic salts by infusing thermally stable acid such as sulfuric acid. Wang et al.42 observed increased yield of aromatic hydrocarbons catalytic pyrolysis of cellulose after infusion with sulfuric acid. In catalytic pyrolysis of acid-infused sewage sludge, sulfuric acid stoichiometrically reacted with the inorganic salts in sewage sludge to form thermally stable sulfates leading to lower yield of carbonaceous residue and higher hydrocarbon

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CONCLUSIONS This study investigated the production of hydrocarbons and ammonia in catalytic pyrolysis (CP) of sewage sludge and the improvements derived from acid pretreatments in a microfurnace pyrolyzer. The primary conclusions from this study are as follows. (1) Pyrolysis temperature had a significant role in the production and selectivity of aromatic hydrocarbons, olefins, and alkanes in catalytic pyrolysis of sewage sludge. High temperature facilitated the vapor release and increased the hydrocarbon production. (2) Both acid-infusion and acid-washing enhanced hydrocarbon production in catalytic pyrolysis of sewage sludge. For acid-washing, by removing the metals out of sewage 1823

DOI: 10.1021/acssuschemeng.6b00016 ACS Sustainable Chem. Eng. 2016, 4, 1819−1826

Research Article

ACS Sustainable Chemistry & Engineering

Table 2. Product Distribution from Catalytic Pyrolysis of Control, Acid-Infused, and Acid-Washed Sewage Sludge at 650 °Ca sewage sludge

control

carbon monoxide carbon dioxide residues aromatic hydrocarbons benzene toluene xylene C9 aromatics C10+ aromatics olefins ethylene propene butene alkanes methane ethane propane butane

8.5 ± 0.1 8.4 ± 0.2 33 ± 0.4 24 ± 0 4.7 ± 0.1 7.9 ± 0 6±0 1.9 ± 0 3.6 ± 0 12.6 ± 0.1 5.8 ± 0 4.8 ± 0 2 ± 0.1 8.7 ± 0.3 4.4 ± 0.3 1.7 ± 0.1 1.3 ± 0 1.3 ± 0

benzene toluene xylene C9 aromatics C10+ aromatics

19.4 ± 0.4 32.7 ± 0.2 25.1 ± 0.2 7.9 ± 0.1 14.8 ± 0

ethylene propene butene

46.6 ± 0.2 37.9 ± 0.1 15.5 ± 0.3

methane ethane propane butane

50.7 ± 1.9 19.5 ± 0.1 15.3 ± 1 14.6 ± 1

acid-infusedb

acid-washed

Overall Yield (C%) 8.8 ± 0.1 8.2 ± 0.2 28.8 ± 0.5 25.3 ± 0.1 5 ± 0.1 8.2 ± 0.1 6.3 ± 0 2.1 ± 0 3.8 ± 0 13.4 ± 0.1 6.3 ± 0 5.1 ± 0 2±0 9.1 ± 0.3 4.4 ± 0.1 1.9 ± 0.2 1.3 ± 0 1.5 ± 0.1 Aromatic Selectivity (%) 19.9 ± 0.3 32.2 ± 0.3 24.9 ± 0.1 8.2 ± 0.1 14.8 ± 0.1 Olefin Selectivity (%) 47 ± 0.1 38.4 ± 0.1 14.6 ± 0.2 Alkane Selectivity (%) 48.8 ± 2.2 20.8 ± 1.7 13.9 ± 0.6 16.5 ± 1.1

acid-infusedc

acid-infusedd

8.7 ± 0.2 9.1 ± 0.3 33 ± 0.4 24.1 ± 0.1 5 ± 0.1 7.9 ± 0.1 6±0 1.7 ± 0 3.4 ± 0 12.3 ± 0.1 5.8 ± 0.1 4.7 ± 0 1.7 ± 0 9.4 ± 0.3 4.7 ± 0.1 2 ± 0.3 1.3 ± 0.1 1.5 ± 0

9.7 ± 0.2 10.9 ± 0.1 22.1 ± 0.4 26.6 ± 0 5.5 ± 0.1 8.8 ± 0.1 6.7 ± 0 2±0 3.6 ± 0 14.1 ± 0.1 6.6 ± 0.1 5.3 ± 0 2.1 ± 0 10.8 ± 0.4 5.5 ± 0.2 2.3 ± 0.1 1.4 ± 0.1 1.5 ± 0.1

9.9 ± 0.1 9.8 ± 0.3 22.3 ± 0.2 26.6 ± 0 5.4 ± 0 8.7 ± 0.1 6.6 ± 0 1.9 ± 0 3.9 ± 0 14 ± 0.1 6.7 ± 0.1 5.4 ± 0 1.8 ± 0 10 ± 0.5 5 ± 0.1 2.2 ± 0.3 1.4 ± 0 1.3 ± 0.2

20.6 ± 0.3 32.8 ± 0.3 25.1 ± 0.1 7.3 ± 0 14.2 ± 0

20.5 ± 0.3 33.2 ± 0.3 25.3 ± 0.1 7.4 ± 0.1 13.7 ± 0

20.5 ± 0.2 32.7 ± 0.4 24.8 ± 0.2 7.3 ± 0 14.8 ± 0

47 ± 0.1 38.7 ± 0.1 14.3 ± 0.2

47 ± 0.3 38 ± 0.2 15 ± 0.1

47.8 ± 0.1 39 ± 0.1 13.3 ± 0.2

49.7 20.8 13.8 15.7

± ± ± ±

0.8 2.4 1.3 0.3

51.3 21.3 13.3 14.1

± ± ± ±

0.4 1.7 0.5 0.9

50.4 21.9 14.4 13.4

± ± ± ±

3.1 2.4 0.8 1.5

“± number” refers to standard deviation. b0.5 mmol sulfuric acid/g (sewage sludge). c1.0 mmol sulfuric acid/g (sewage sludge). d2.0 mmol sulfuric acid/g (sewage sludge).

a

sludge sample, the formation of solid residue during catalytic pyrolysis process was weakened and more volatiles were released to form hydrocarbons. For acidinfusion, the reactive metals were passivated by sulfuric acid to form thermally stable sulfates in pyrolysis and more volatiles were released to form hydrocarbons. Acidinfusion was better than acid-washing with an optimal sulfuric acid loading of 1 mmol·g−1 of sewage sludge at 650 °C. (3) Catalytic pyrolysis of acid-pretreated sewage sludge enhanced the denitrogenation of volatiles by releasing nitrogen as ammonia which could be recycled as fertilizer to increase its economic feasibility.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +1 515 294 0913. Fax: +1 515 294 8993. E-mail address: [email protected] (M.M.W.). *Tel.: +86 451 86283017. Fax: +86 451 86282100. E-mail addresses: [email protected] (Q.Z.).

Figure 7. Nitrogen distributions of pyrolysis of sewage sludge, acidinfused sewage sludge (1 mmol·g−1) and acid-washed sewage sludge with/without catalyst at 650 °C.

Notes

The authors declare no competing financial interest. 1824

DOI: 10.1021/acssuschemeng.6b00016 ACS Sustainable Chem. Eng. 2016, 4, 1819−1826

Research Article

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ACKNOWLEDGMENTS The authors acknowledge the support of the NSF EPSCoR Grant EPS-1101284 and the financial support of China Scholarship Council. This work was made possible by the experimental facilities and research staff from the Bioeconomy Institute at Iowa State University.

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DOI: 10.1021/acssuschemeng.6b00016 ACS Sustainable Chem. Eng. 2016, 4, 1819−1826