Thermocatalytic Studies on Municipal Solid Waste

Thermocatalytic Studies on Municipal Solid Waste

Available online at www.sciencedirect.com ScienceDirect Energy Procedia 105 (2017) 706 – 711 The 8th International Conference on Applied Energy – IC...

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Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 105 (2017) 706 – 711

The 8th International Conference on Applied Energy – ICAE2016

Thermocatalytic studies on municipal solid waste Z. Sebestyéna*, N. Miskolczib, E. Barta-Rajnaia, E. Jakaba, Zs. Czégénya a

Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Magyar Tudósok körútja 2, H-1117 Budapest, Hungary b University of Pannonia, Institutional Department of MOL Hydrocarbon and Coal Processing, Egyetem utca 10, H-8201 Veszprém, Hungary

Abstract The goal of this study was to upgrade the decomposition products of municipal solid waste applying different catalysts during slow and high heating rate pyrolysis. The municipal waste studied was a mixture of biomass and plastics in a ratio of 1:1. Different zeolites (E- and Y-zeolites, ZSM-5 and FCC catalysts) and nickel-molybdenum (NiMo/Al2O3) catalysts were tested in order to upgrade the quality of the decomposition product mixture. The applied sample-catalyst ratio was 2:1. The evolution profiles of the gas phase decomposition products were monitored by thermogravimetry/mass spectrometry (TG/MS). The product distribution was analyzed by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) technique. The effect of the catalysts on the biomass part of the waste sample is very similar. The thermal decomposition of the plastic fraction is catalyzed mainly by E-zeolite and ZSM-5, as the thermogravimetric and the mass spectrometric data indicate. The pyrolysis results unambiguously show the cracking effects of the applied catalysts on the composition of the evolving hydrocarbon mixture. The yield of the aromatic products is increased significantly. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). under responsibility of ICAE Selection and/or peer-reviewof Peer-review under responsibility the scientific committee of the 8th International Conference on Applied Energy.

Keywords: biomass-plastic waste, thermogravimetry/mass spectrometry, catalytic pyrolysis, E-zeolite, ZSM-5, FCC catalyst

1.

Introduction

The composition of the municipal solid waste depends on the living standards and the lifestyle of the human population. The organic part of the municipal solid waste can be universally divided into two main fractions: biomass and plastics. The energy content of biomass-plastics waste can be utilized by thermochemical methods. Co-pyrolysis and co-gasification of biomass and plastic materials has already been communicated by many papers [1-3]. The main advantage of these techniques is that there is no need for the expensive separation processes. The waste samples can be converted to combustible gas mixture, liquid bio-oil and carbonaceous residue. The pyrolysis oil with high energy content is the most promising energy carrier among the pyrolysis products. However the quality of the oil is reduced by the oxygen-containing organic molecules of * Corresponding author. Tel.: +36-1-382-6510 E-mail address: [email protected]a.hu

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.379

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biomass origin. Moreover these molecules are susceptible to the polymerization so the oil originating from biomass materials is unstable [4]. The quality of the pyrolysis oil and the energy content of the pyrolysis gas mixture can be upgraded using different types of catalysts. Microporous beta and Y types of zeolites are generally applied for liquid phase esterification in the biodiesel production process [5,6]. ZSM-5 and FCC catalysts are commonly used for catalytic conversion in fluidized bed reactors [7,8]. Nickel-molybdenum (NiMo/Al2O3) catalyst is often applied for enhancing the yield of the molecular hydrogen in various reactions [9]. In this work the pyrolysis of municipal solid waste was investigated using various catalysts. The goal was to screen the potentials of different catalysts on the thermal recycling of municipal solid waste. The catalytic pyrolysis processes using slow and high heating rates have been studied aiming further utilization and possible upgrading of the thermal decomposition products. 2. Experimental section 2.1 Municipal solid waste mixture The studied municipal solid waste sample was obtained from a local waste management company (Pécs, Hungary). One portion of the waste was carefully sorted to plastic and biomass part. The measured biomass:plastic ratio was 1:1 by weight. The unseparated part of the sample was cryo-milled in a Retsch MM301 mill, to obtain homogenous sample. 2.2 Catalysts Different zeolites: E-zeolite, Y-zeolite, ZSM-5, FCC (fluid catalytic cracking) and nickel-molybdenum (NiMo/Al2O3) catalysts were applied and the catalytic effect was studied on the quality of the pyrolysis oil and the gas mixture. Sample:catalyst ratio of 2:1 was applied during the measurements. The most important properties of the applied catalysts are given in the Table 1. Table 1. The most important properties of the applied catalysts

Catalyst Y-zeolite ZSM-5 E-zeolite NiMo/Al2O3 FCC

BET surface (m2/g) 229 350 395 158 210

Surface of the micropores (m2/g) 20 110 29 215 35 259 n.d. 74 17 90 n.d: not determined Si/Al ratio

Acidity (NH3/g) 0.38 0.44 0.48 0.30 0.30

2.3 Thermogravimetry/mass spectrometry (TG/MS) The TG/MS system consists of a modified Perkin-Elmer TGS-2 thermobalance and a Hiden HAL quadrupole mass spectrometer. About 1.2 mg municipal waste samples were measured in argon atmosphere at a flow rate of 140 ml min-1. Approximately 1.8 mg sample amounts were applied using the mixture with catalysts mixed in the ratio of waste:catalyst 2:1. The samples were heated in a platinum sample pan at a rate of 10 °C min-1 from 25 to 700 °C. The evolved products were introduced through a glass lined metal capillary heated at 300 °C into the ion source of the mass spectrometer which was operated at 70 eV electron energy.

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2.4 Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) Approximately 1.4 mg samples were pyrolyzed at 550 °C for 20 s in helium atmosphere using a Pyroprobe 2000 pyrolyzer interfaced to an Agilent 6890A/5973 GC/MS. The sample sizes were about 2 mg in case of the biomass-plastic and catalyst mixtures. This rather high sample amount was chosen for the pyrolysis experiments to make the contact possible between the sample and the catalyst. The pyrolysis products were separated on a DB-1701 capillary column (30 m × 0.25 mm, 0.25 Pm film thickness). The GC oven was programmed to hold at 40 °C for 4 min then increase the temperature at a rate of 6 °C min-1 to 280 °C (hold for 5 min). The mass range of m/z 14-500 was scanned by the mass spectrometer in electron impact mode at 70 eV. Two or three replicates were carried out with each sample. 3. Results and discussion 3.1 TG/MS results Fig. 1 shows the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of the municipal waste sample and the sample mixed with E- and Y-zeolite, FCC, NiMo/Al2O3 and ZSM-5 catalysts. The TG (Fig. 1a) and DTG (Fig. 1b) curves show that the thermal decomposition of the waste sample can be divided into two main steps. According to the Py-GC/MS experiments discussed below the first step can be attributed to biomass and the second step to plastic decomposition. The decomposition of the biomass part of the sample starts at about 200 °C and ends at around 390 °C. The highest DTG peak at about 350 °C represents the thermal decomposition of cellulose and the characteristic shoulder on the DTG curve belongs to the mass loss owing to the degradation of hemicellulose fraction. The thermal decomposition of biomass is particularly catalyzed by NiMo/Al2O3, FCC and E-zeolite catalysts. The second DTG peak between 390 and 510 °C can be attributed to the decomposition of the plastics composed of mainly polyethylene and polypropylene (as determined by Py-GC/MS). The decomposition of the plastic part shifts to a lower temperature range applying the E-zeolite and ZSM-5 catalysts.

Figure 1. (a) TG and (b) DTG curves of municipal solid waste sample with and without the catalysts On the TG curves (Fig. 1a) it can be seen that the highest yield of the carbonaceous residue was formed from the sample mixed with the ZSM-5 catalysts. The char yield decreased by the application of all other catalysts studied.

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Fig. 2 shows the TG/MS curves of the waste sample without catalyst (2a) and with E-zeolite (2b). According to the DTG curves E-zeolite has impacts on both components of the municipal waste. The cracking effect of E-zeolite can be observed by comparing the evolution profiles of the volatile decomposition products in the absence and in the presence of the catalyst (Fig. 2a and b). Formaldehyde is a general product of cellulose, hemicellulose and lignin represented by the ion curve of CHO+ fragment ion (m/z 29) at the temperature range of biomass decomposition. At higher temperatures (between 420 and 520 °C), the m/z 29 ion curve describes the evolution profile of C2H5+ fragment of hydrocarbons originating from the decomposition of PE and PP. The increased yield of aldehyde and ethyl groups in Fig. 2b indicates the catalytic effect of E-zeolite on biomass and plastic fraction as well. The yields of alkyl (CH3- and C2H5-) and alkenyl groups (C2H3- and C3H5-) increased significantly in the presence of the catalyst. The intensity of the propylene groups (m/z 41) is four-times higher when applying the zeolite. Propylene monomers may be formed by cracking the propylene trimer, which is a characteristic pyrolysis product of polypropylene. This explanation is confirmed by the pyrolysis-GC/MS data discussed in the next section, which shows decreasing yield of the PP-trimer (2,4-dimethyl-1-heptene) in the presence of E-zeolite, while the yield of propylene monomer was simultaneously doubled in the chromatogram.

Figure 2. DTG curves and evolution profiles of some gaseous products (methyl group – m/z 15, ethenyl group – m/z 27, aldehyde and ethyl groups – m/z 29 and propylene group – m/z 41) derived from municipal solid waste (a) without catalysts and (b) in the presence of E-zeolite catalyst

3.2 Py-GC/MS results Pyrolysis-gas chromatography/mass spectrometry has been applied to reveal the changes in the pyrolysis product distribution of the municipal solid waste sample with and without the catalysts. The pyrolysis temperature was chosen 550 °C because the TG results showed that the thermal decomposition of the waste sample ends slightly above 500 °C (Fig. 1). During pyrolysis a lot of volatile compounds are formed, which is difficult to evaluate. Therefore the pyrolysis products were grouped into the major chemical classes. Table 2 contains the summed total ion chromatogram area of the main classes of the decomposition products. The increased yield of the gas phase products e.g. carbon dioxide, methane and propylene indicates the cracking effect of the applied catalysts. The gas yield increased by 50% applying the E-zeolite; meanwhile the yields of polyethylene (alkanes, 1-alkenes and dienes) and polypropylene (dimer, trimer and higher oligomers of the PP)

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decomposition products decreased significantly. Levoglucosan (1,6-anhydro-E-D-glucopyranose) is the main pyrolytic decomposition product of cellulose. Among the anhydrosugars xylofuranose and mannopyranose molecules can be found as well; originating from the hemicellulose component of the plant biomass. The decreased yield of the anhydrosugar molecules indicates the catalytic effect of all catalysts on the biomass fraction. Table 2. Relative intensities (summed total ion chromatogram area %) of the major groups of the pyrolysis products of municipal solid waste without and in the presence of the catalysts

Compounds’ groups Gas

Area % Waste

Area % Waste: Area % Waste: Area % Waste: Area % Waste: Area % Waste: Y-zeolite ZSM-5 FCC NiMo/Al2O3 E-zeolite

17.0

25.9

20.7

23.6

19.0

18.9

37.1

22.7

35.2

29.0

39.2

33.7

9.0

7.0

5.3

6.4

3.9

6.5

Anhydrosugars

13.6

12.6

9.4

11.6

10.2

12.0

Acetic acid and hydroxyacetaldehyde

1.7

1.1

1.3

1.0

1.7

1.8

Aromatics

14.8

24.3

21.9

21.5

20.3

19.5

Others

6.8

6.4

6.3

7.0

5.8

7.6

Polyethylene (PE) decomposition products Polypropylene (PP) decomposition products

Acetic acid and hydroxy-acetaldehyde are characteristic decomposition products of the hemicellulose and cellulose, respectively. The summed yield of these products decreased by 25-40% in the presence of 3 zeolite catalysts, which is a beneficial effect because this change reduces the polarity and the acidity of the pyrolysis oil. During the pyrolysis of the biomass-plastic waste about one seventh (14.8%) of the decomposition products are aromatics. As a result of the catalysts application, the amounts of the aromatics were increased by 30-60%. These molecules may originate from the lignin part of biomass from the long PE and PP chains by catalytic aromatization. 4. Conclusion In this study the possible upgrading of the thermal decomposition products was studied. The influence of various catalysts on the thermal decomposition of a municipal solid waste sample was measured by TG/MS and Py-GC/MS in order to contribute to the better utilization of its energy content. The TG results revealed that the biomass decomposition was slightly catalyzed by most of the applied catalysts, while ZSM-5 and E-zeolite had more significant effect on the thermal decomposition of the plastic component of the mixture comparing to other catalysts. The cracking effects of the catalysts were monitored by the MS evolution profiles of hydrocarbons of smaller molecular mass. In the presence of the catalysts, the intensities of the smaller hydrocarbon fragments have been considerably enlarged. The decreased yield of the PP-trimer and the increased amount of propylene molecules in the gas phase products demonstrate the cracking effect of the E-zeolite. The total gas yield has grown by 50 %. The catalysts promote the disruption of the long PE, PP and sugar chains. The most beneficial effect of the catalyst application is the increasing yield of the aromatics which improves the octane rating of the oil. Another advantage of ZSM-

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5, E- and Y-zeolites is that they reduce the yield of acetic acid and hydroxyacetaldehyde hereby improving the stability of the pyrolysis oil. Acknowledgements This work was supported by NKFIH, Hungary & DST, India through the Bilateral Cooperation between Hungary (project No. TÉT_13_DST-1-2014-0003) and DST-India (DST/INT/HUN/P-02/2014) and “Bolyai János” research fellowship. References [1] Sajdak M, Slowik K. Use of plastic waste as a fuel in the co-pyrolysis of biomass: Part I. The effect of the addition of plastic waste on the process and products. J. Anal. Appl. Pyrolysis 2014;107:267-75. [2] Xue Y, Zhou S, Brown RC, Kelkar A, Bai X. Fast pyrolysis of biomass and waste plastic in a fluidized bed reactor. Fuel 2015;156:40-6. [3] Pinto F, Franco C, André RN, Miranda M, Gulyurtlu I, Cabrita I. Co-gasification study of biomass mixed with plastic wastes. Fuel 2002;81:291-7. [4] Letho J, Oasmaa A, Solantausta Y, Kytö M, Chiaramonti D. Review of fuel oil quality and combustion of fast pyrolysis bio-oils from lignocellulosic biomass. Appl. Energy 2014;116:178-90. [5] Yue Y, Liu H, Zhou Y, Bai Z, Bao X. Pure-phase zeolite beta synthesized from natural aluminosilicate minerals and its catalytic application for esterification. Appl. Clay Sci. 2016;126:1-6. [6] Doyle AM. Albayati TM, Abbas AS, Alismaeel ZT. Biodiesel production by esterification of oleic acid over zeolite Y prepared from kaoline. Renew.Energy 2016;97:19-23. [7] Zhang X, Lei H, Zhu L, Qian M, Zhu X, Wu J, Chen S. Enhancement of jet fuel range alkanes from co feeding of lignocellulosic biomass with plastic via tandem catalytic conversions. Appl. Energy 2016;173:418-430. [8] Zhang Y, Yu D, Li W, Gao S, Xu G, Zhou H, Chen J. Fundamental study of cracking gasification process for comprehensive utilization of vacuum residue. Appl. Energy 2013;112:1318-25. [9] Jinlong L, Tongxiang L, Chen W. Investigation of hydrogen evolution activity for the nickel, nickel-molybdenum nickelgraphite composite and nickel reduced graphene oxide composite coatings. Appl.Surf. Sci. 2016;366:353-8.

Biography Zoltán Sebestyén is working for the Institute of Materials and Environmental Chemistry in the Research Centre for Natural Sciences of the Hungarian Academy of Sciences as a research fellow. His research field of interest is the thermal decomposition processes of lignocellulosic and protein based biomass materials.

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