Thermal Reduction of NOx with Recycled Plastics - ACS Publications

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Thermal Reduction of NOx with Recycled Plastics

Ibukun Oluwoye,† Bogdan Z. Dlugogorski,*,† Jeff Gore,‡ Sergey Vyazovkin,§ Olivier Boyron,∥ and Mohammednoor Altarawneh† †

School of Engineering and Information Technology, Murdoch University, 90 South Street, Murdoch, Western Australia 6150, Australia ‡ Dyno Nobel Asia Pacific Pty Ltd, Mt Thorley, New South Wales 2330, Australia § Department of Chemistry, University of Alabama at Birmingham, 901 S. 14th Street, Birmingham, Alabama 35294, United States ∥ Laboratoire de Chimie Catalyse Polymères et Procédés (C2P2), Université Claude Bernard Lyon 1, UMR CNRS 5265, 43 bd 11 Novembre 1918, F-69616 Villeurbanne Cedex, France S Supporting Information *

ABSTRACT: This study develops technology for mitigation of NOx formed in thermal processes using recycled plastics such as polyethylene (PE). Experiments involve sample characterization, and thermogravimetric decomposition of PE under controlled atmospheres, with NOx concentration relevant to industrial applications. TGA−Fourier transform infrared (FTIR) spectroscopy and NOx chemiluminescence serve to obtain the removal efficiency of NOx by fragments of pyrolyzing PE. Typical NOx removal efficiency amounts to 80%. We apply the isoconversional method to derive the kinetic parameters, and observe an increasing dependency of activation energy on the reaction progress. The activation energies of the process span 135 kJ/mol to 226 kJ/mol, and 188 kJ/mol to 268 kJ/mol, for neat and recycled PE, respectively, and the so-called compensation effect accounts for the natural logarithmic pre-exponential ln (A/min−1) factors of ca. 19−35 and 28−41, in the same order, depending on the PE conversion in the experimental interval of between 5 and 95%. The observed delay in thermal events of recycled PE reflects different types of PE in the plastic, as measurements of intrinsic viscosity indicate that, the recycled PE comprises longer linear chains. The present evaluation of isoconversional activation energies affords accurate kinetic modeling of both isothermal and nonisothermal decomposition of PE in NOx-doped atmosphere. Subsequent investigations will focus on the effect of mass transfer and the presence of oxygen, as reburning of NOx in large-scale combustors take place at higher temperatures than those included in the current study.

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INTRODUCTION Apart from its limited industrial and medical applications, oxides of nitrogen (NOx) contribute to acid rain, ecoeutrophication, photochemical smog, ground level ozone, greenhouse effect, stratospheric ozone depletion, and health deterioration (e.g., chronic respiratory and obstructive pulmonary dysfunction) of predisposed organisms. Primarily, NOx consists of nitric oxide (NO) and nitrogen dioxide (NO2). Typical anthropogenic emission of NOx (comprising mostly NO) amounts up to 800 ppm in untreated combustion systems burning fossil/nitrogenous fuels for power generation, and can soar as high as 500 ppm during decomposition of nitrate oxidizers in some aviation propellants and mining-grade explosives.1−11 The quest for reliable NO x reduction has led to implementation of strict environmental regulations and development of industrial-scale technologies. In combustion systems, these technologies include fuel pretreatment, combustion modification and post-treatment techniques.12−14 © XXXX American Chemical Society

Although postflame gas cleanup remains viable, strategic modification of combustion processes often controls NOx emission more economically.2,15,16 Noticeably, fuel reburning provides a commercially available technological retrofit that appears more effective than alternative modifications of combustion processes. The process abates NOx by using fuel as a reducing agent.17 Since 1950s, a number of practical and experimental studies investigated the reburning technique,4,18−24 including a collateral effect of SO2 reduction.25 Conceptually, reburning occurs in three stages (see Figure 1) to achieve between 50% and 85% reduction in the total NOx emission. In the first stage (primary zone), the main fuel burns under slightly fuel-lean condition. Afterward, the produced NOx reacts with supplementary hydrocarbon radicals Received: Revised: Accepted: Published: A

November 4, 2016 May 4, 2017 May 31, 2017 May 31, 2017 DOI: 10.1021/acs.est.6b05560 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

reduce NOx emission in industrial activities. We aim at solving two crucial environmental problems, to develop a sustainable solution to NOx emission, and to provide additional means of dealing with excessive plastic wastes in the society. In particular, we report the result of thermal decomposition of PE in NOxdoped atmosphere, providing a detailed analysis of reaction products. The insights from the present study apply equally to abatement of NOx with materials laden with PE, polyolefins/ paraffins and pure carbon−hydrogen-type polymers, as well as to cocombustion/pyrolysis of PE with nitrogen-rich fuels such as biomass and coal.

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EXPERIMENTAL SECTION Materials and Sample Characterization. The experiments involved recycled polyethylene received in form of Table 1. HT-SEC Parameters of Neat and Recycled PE Samples: Peak-Average, Number-Average and WeightAverage Molar Masses (Mp, Mn, and Mw), As Well As Intrinsic Viscosity [η], Radius of Gyration (Rg) and Dispersity Đ in TCB Solution

neat PE recycled PE

Figure 1. Schematic diagram of the three stages of the reburning process.

in the “reburning zone” to form intermediate nitrogenous species, and then molecular nitrogen (N2). Finally, addition of air in the last stage (burnout zone) completes the combustion process by oxidizing all unreacted fuels and reduced Nspecies.2,4 Although, large-scale reburning systems usually rely on hydrocarbon gases (e.g., methane), solid carbonaceous residues have proven equally suitable. Spliethoff et al. employed the pyrolysate fragments of coal as a reburn fuel to mitigate NOx formation in a bench-scale reburning facility, and attained good reduction efficiency of about 80%.26 Likewise, demonstrations of application of coal reburning in commercial boilers achieved excellent NOx control, averaged at around 70% reduction efficiency.27−30 Biomass fuels such as wood, straw, rice husk, bio-oil, sewage sludge, and carbonized municipal solid waste also provide effective means of reducing NOx formation (50− 75%) in combustion systems.31−40 Yet surprisingly, only a few studies assessed the performance of polymeric materials (e.g., waste tires 41−43 ) as a secondary reburn fuel for NO x remediation. Unlike coal or biomass, plastics, such as polyethylene (PE), lack nitrogen functionalities that can further contribute to fuelNOx formation within the reburning zone. Owing to its high calorific value, good volatile content and favorable combustion rate,44−46 waste PE finds outlets in recycling processes such as thermal energy recovery, as it burns without prompting environmental concerns, for example, formation of pollutants and fire-related hazards.47,48 Measurements of pyrolytic conversion of PE to low-carbon gases in hybrid rocket propulsion49,50 indicate the suitability of the polymer in reburning applications, and as a potential NOx reductant during detonation of ammonium nitrate-based explosives in open-cut mining operations. To this end, the current study presents a series of experimental investigations of thermal mitigation of NOx with PE. We hypothesize that, waste PE can potentially serve to

Mp (kg/mol)

Mn (kg/mol)

Mw (kg/mol)

[η] (dL/g)

Rg(w) (nm)

Đ= Mw/Mn

59.5 77.1

14.2 24.4

144 105

1.1 1.9

31.6 34.9

10.1 4.3

Figure 2. FT-IR spectrum of the recycled polymer.

Table 2. Elemental Characterization of the Recycled Polymer; Basic Elements from a CHNS Analyzer, Halogens from IC and Trace Metals from ICP-OES. Error Margin for CHNS Analysis is 0.3% ultimate elements and halogens (wt %) C

H

86.17

14.23

N

S

Cl

0.00 0.00 0.10 average trace metals (wt %)

Br

F

0.00

0.00

Al

Ca

Fe

Mg

Mn

Na

Fe

0.10

0.24

0.03

0.05

0.02

0.11

0.25

extruded black cylindrical pellets, around 2.5 mm in diameter and 2 mm in length. We sliced the pellets into thin oblong particles of uniform size distribution of between 250 and 400 μm. Sigma-Aldrich, provided neat low-density polyethylene, employed in control studies, to gauge the effect of impurities in B

DOI: 10.1021/acs.est.6b05560 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Article


Figure 3. Schematics of adopted experimental configurations. A standalone thermogravimetry apparatus (TGA, top) eliminates any unwanted experimental artifacts that could be carried over into the subsequent kinetic evaluations. Hyphenated TGA−FTIR (middle) and TGA−NOx analyzer (bottom) enabled the identification and quantitation of evolved gases.

the recycled PE on its thermal and NOx-abatement behavior. The attenuated total reflectance (ATR) produced the IR spectra of the recycled PE, and a carbon-hydrogen-nitrogensulfur (CHNS) elemental analyzer yielded the elemental composition of the material. Further analyses comprised the multidetector high temperature size exclusion chromatography (HT-SEC), ion (anion) chromatography (IC), and inductively coupled plasma optical emission spectroscopy (ICP-OES) for determination of molecular mass distributions (MMD), halogens and trace metals, respectively. Supporting Information (SI) includes details of these analytical techniques. Table 1 itemizes the molecular characteristics of the PE samples. The FTIR spectrum of the recycled PE, documented in Figure 2,

indicates no contamination with other plastics, and Table 2 reveals the combined amount of metals and halogens, mostly Fe, Ca and Cl, of less than 1%. The recycled PE contains no sulfur, nitrogen, fluorine and bromine-containing contaminants, confirming the absence of brominated flame retardants in the sample. Experimental Methods. Figure 3 illustrates the complete schematics of the three arrangements of the experimental apparatus. A carefully calibrated (by melting point standards) PerkinElmer STA 8000 system facilitated all thermogravimetric analyses (TGA), with the quantitation of the gases performed by infrared spectroscopy (TGA-FTIR, PerkinElmer Frontier) and chemiluminescence (TGA-NOx, Thermo Scientific model C


Article

Environmental Science & Technology 42i-HL). All TGA experiments involved three replicate runs (SI Figure S4) of 5.2 ± 0.2 mg samples, placed in alumina crucible, subjected to the nominal heating rates of 5, 10, 15, and 20 K/ min, bracketing temperatures between 25 to 600 °C in argon, synthetic air and NOx (610 ± 5 ppm of NO and 18 ± 3 ppm of NO2 with balance of dry helium gas) atmosphere. The continuous flow rate of the purge gases amounted to 80 mL/ min, as measured under the standard temperature and pressure (STP). Blank runs completed under identical conditions to the production experiments yielded the thermal buoyancy corrections. We have achieved excellent reproducibility of the TGA results (SI Figure S4). The TGA-FTIR and TGA-NOx measurements required larger sample sizes of 10.2 ± 0.2 mg to improve the detection and quantitation of gaseous species. The FTIR spectrometer averaged two accumulated scans per spectrum at 1 cm−1 resolution. This resulted in a temporal resolution of 30 s, which we found sufficient even for the fastest temperature TGA ramp of 20 K/min. The TGA-FTIR setup incorporated an electrically heated transfer line (PerkinElmer TL 8500) linking the TGA system to 100 mm path-length online gas sampling cell placed inside the FTIR compartment. Both the sample line and the sampling cell operated at 220 °C, to avoid the condensation of product volatiles prior and during the analysis. An externally controlled vacuum pump maintained a balanced flow throughout the system. The lag time between the TGA and FTIR corresponded to 10 s, requiring a time-delay correction for comparison of the mass-loss and the evolved gas-composition signals.

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RESULTS AND DISCUSSION Thermal Decomposition of PE in NOx Atmosphere and Evolved Gas Analysis. Figure 4 presents the characteristic thermogravimetric measurements of neat and recycled PE, as the samples decompose under Ar, He-NOx and air atmospheres. In agreement with previous studies, in inert argon purge, the pyrolysis proceeds in an apparent single-step process that commences at around 446 °C (extrapolated onset temperature), attains a maximum at around 477 °C, and then rapidly slows down owing to depletion of the material, to cease by 490 °C (extrapolated offset temperature). The residual char and ash constitutes about 0.8% for neat and 2% for the recycled PE, respectively. Furthermore, the presence of oxygen triggers multistep, low-temperature, peroxy-channelled reactions, because of O2 addition to radical sites in the polymer chains.51 Under an inert shield gas, PE decomposes into a series of lower molecular weight hydrocarbons, mostly linear n-alkanes and n-alkenes,48,50,52 and this can be monitored by the series of CHi bands, most easily at 2922 cm−1 that corresponds to the CH2 asymmetric stretch; see Figure 5. Other evolved functional group characteristics include CH3 asymmetric and symmetric stretches, C−H out-of-plane bend, CH2 rocking vibration and symmetric stretch, as well as CH3 asymmetric bend. These species emerge within the temporal range, corresponding to ca. 400−500 °C, correlating well with the thermal decomposition period of the samples shown in Figure 4. The presence of NOx, delivered in helium bath gas, accelerates the thermal decomposition of PE, resulting in the maximum rate of pyrolysis shifting from 477 °C (or 479 °C in the case of recycled PE) to 471 and 474 °C, for neat and recycled PE, in that order. This acceleration occurs due to the oxidative nature of NOx. The onset and completion temperatures, under He-NOx purge, correspond to 433 and 483 °C as

Figure 4. TGA curves for the thermal decomposition of neat (a) and recycled (b) PE in different purge atmospheres at a nominal heating rate of 10 K/min. Inset plots magnify the respective normalized derivatives of the thermal events. NOx denotes a mixture of 610 ppm of NO and 18 ppm of NO2 in balance of dry helium.

well as 444 and 486 °C, for neat and recycled PE, respectively. The observed delay in the thermal events of recycled PE suggests different molecular branching structures53 (SI Figure S1) as well as possible confining activity of the constituent inorganic residues. Inorganic residues can form microglassy layers that may become impenetrable to volatiles, hence inhibit the thermal breakdown of the underlying organic matrices.54 As shown in Figure 6,55 below the decomposition temperature, the barrierless exothermic addition of NO and NO2 follows the formation of radical sites in condensed-phase PE. This process results in formation of C-nitroso and nitro species, respectively, and leads to internal tautomerisation or concerted elimination of nitroxyl (HNO) and nitrous acid (HNO2). This set of reactions can disrupt the C−C bonding configurations, and in turn lower the overall enthalpic requirements for the homolysis of the C−C bond and unzipping of the polymer in the successive β-scission reactions.56 The peak value of 2922 cm−1 (Figure 5) reduces substantially when similar amount (10 mg) of PE decomposes under NOx atmosphere, signifying decrease in concentration of CHi fragments and hence, gas-phase reactions between the PE pyrolysates and the NOx gases. This interaction involves several chain steps that can be globalized as4,40

∑ CiHj + NO → HCN + ... D

(1) DOI: 10.1021/acs.est.6b05560 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Figure 5. IR monitoring of hydrocarbon bands evolved during thermal decomposition of PE samples (10 mg) in two purge atmospheres at nominal heating rate of 20 K/min. The four inset plots represent the maximum evolution events with respect to absorbance values, at different time t. List of observed absorbance peaks (cm−1) − 2922: CH2 asymmetric stretch; 2850: CH2 symmetric stretch; CH3 asymmetric bend; 915/990: out-of-plane C−H bend; 723: CH2 rocking vibration. CH2 bands overlap with the CH3 asymmetric stretch at 2962 and the symmetric stretch at 2873.

In the absence of fuel-N in PE, the other NH3 pathways remain dormant. The detection limit of the employed gas cell (100 mm path-length and 11.3 cm3 volume) restrains the online elucidation of nitrogenous species. However, the chemiluminescence analyzer monitors the NO/NOx profile ensuing under the dynamic thermal conditions. As illustrated in Figure 7 (as well as SI Figures S11 and S12), within the decomposition regime of the polymer, the experimental measurements show significant reduction of NOx up to 80%. The heating ramp affects the reaction via the Arrhenius term in the rate expression,57 dictating the composition of the volatile species.48,50,58 Even though the moderate heating rates alter the NOx conversion profile in SI Figure S12, the maximum conversion efficiency remains approximately the same (Figure 7b). As explained earlier, although the recycled PE displays a delay in the evolution of volatile fragments, the maximum NOx reduction efficiency remains the same when compared to that of neat PE samples. Degradation Kinetics. As expected, the decomposition of PE exhibits strong dependency on the heating rate (Figure 4 and SI Figure S4). This allows the application of the isoconversional principle,57 to obtain the activation energy as a function of the conversion α. As illustrated in SI, the calculations rely on the advanced isoconversional method of Vyazovkin57,59 (eq 6) that provides an accurate estimation of activation energy values from the temperature integral, as compared to Lyon,60 Kissinger-Akahira-Sunose,61 and Starink’s62 approximate approaches. For a series of runs performed at different heating rates, the appropriate activation energy value minimizes the following function:

Figure 6. Reaction networks for interactions between condensedphase PE and aggressive NOx species. Bolded values represent reaction enthalpies and values in italic correspond to the activation enthalpies. All values (in kJ/mol) relate to respective reactants at 298.15 K.55

In excess NOx, the HCN decays through series of intermediates, and ultimately reaches N2 via the reverse Zeldovich reaction, eq 5: HCN + O → NCO + H

(2)

NCO + H → NH + CO

(3)

NH + H → N + H 2

(4)

N + NO → N2 + O

(5) E


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Figure 7. Typical NOx conversion history during the thermal decomposition of PE samples (10 mg) in NOx atmosphere under a nominal heating rate of 20 K/min (a), and a comparison of maximum possible conversion efficiency as a function of the dynamic temperature ramp (b). The overall weighted mean for NOx conversion corresponds to 81%. The NOx conversion gradually falls to zero as the reaction depletes the fragments of pyrolyzing PE. A response analysis (time-lag corrections) shifts the raw NOx measurements, but does not account for dispersion in the reactor, and between the reactor and the NOx detector. n

⌀(Eα) =

n

∑∑ i=1 j≠i

Figure 8. Kinetic parameters accompanying the thermal decomposition of neat (a) and recycled (b) PE in argon and He-NOx baths. The gray trend-shades depict accuracy thresholds of the evaluated parameters.

I(Eα , Tα , i)βj I(Eα , Tα , j)βi

(6)

The subscripts i and j represent integer numbers of different experiments performed under varying heating programs, that is, i, and j corresponds to 1−4 for heating rates (β), Eα and Tα respectively symbolizes the activation energy and temperature values related to a given extent of conversion (α), and I(E,T) denotes the Arrhenius temperature integral. Furthermore, the so-called compensation effect permits the determination of the pre-exponential factor A by the substitution of integral form of various reaction models g(α) into numerical approximation of eq 6.57,63 Although this method relies on the assumption that the thermal process obeys single-step kinetics, it can also provide a reasonable estimation64 of Aα dependency on α. The use of the Eα and Aα dependencies has allowed for determining the reaction model.57 SI Figure S7 indicates that, the thermal decomposition of PE remains consistent with the Mampel first order model of f(α) = (1-α) over the range of measured activation energies. SI provides detailed discussion of the adopted method to determine the pre-exponential factor A.

Figure 9. Isothermal decomposition of neat (a) and recycled (b) PE under the He-NOx atmosphere at three moderate temperatures. Continuous lines represent experimental results, and dotted points denote the corresponding theoretical predictions (up to α = 0.95) as evaluated from eq 7.

Previous investigators obtained the kinetic parameters for the pyrolysis of neat PE.51 Their reported isoconversional activation energies (150−240 kJ/mol) match with our F



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estimated values of 160−255 kJ/mol. The illustrated growth in Eα (Figure 8a and 8b) is a result of change in reaction mechanism. The initial low value relates to the initiation process occurring at the defect sites in the polymer chain. Eα varies as the limiting step of the degradation process shifts toward other mechanisms, such as unzipping, backbiting, random scission and branching.48,51,65,66 As shown in Figure 8, the presence of ca. 600 ppm of NOx lowers the Eα values for the decomposition of neat PE by 13−18%. Returning to Figure 4, both the neat and recycled PE experience acceleration of degradation (shift to lower temperature) under NO x atmosphere. However, the effect manifests itself differently in the kinetics. For neat PE, the drop in Eα (Figure 8a) represents a lower energy barrier in the presence of NOx, hence reaction is faster. On the contrary, Eα increases in the case of recycled PE, but Figure 4 depicts accelerating reaction. This means that, the mechanism of acceleration for recycled PE differs from that of neat PE. The observed acceleration during decomposition of recycled PE under He-NOx relates to a higher pre-exponential A factor. This factor depends on MMD, in particular on higher number-average molar mass (Mn in Table 1) for recycled PE. The recycled PE comprises longer linear chains (as also evident from the higher value of intrinsic viscosity [η] for recycled polymer in Table 1). The longer the chain that breaks during the reaction with NOx, the higher entropy change, and larger the pre-exponential A factor. The isoconversional kinetic parameters (plotted in Figure 8 and assembled in SI Tables S4 and S5) substituted in eq 767 provide a means to estimate the time (tα) needed to achieve a target conversion α under isothermal condition (T0): tα =

1 β

T

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b05560. Assessments of calibration and repeatability of TGA experiments, thermal characterization of recycled PE, derivation of cited equations, as well as analysis of the compensation effect, complete isoconversional kinetic data, enlarged time-resolved FTIR spectra, and complete NOx reduction trends (PDF)

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*Phone: (+) 61 8 9360 6770; fax: (+) 61 8 9360 6346; e-mail: [email protected]. ORCID

Bogdan Z. Dlugogorski: 0000-0001-8909-029X Sergey Vyazovkin: 0000-0002-6335-4215 Mohammednoor Altarawneh: 0000-0002-2832-3886 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study has been funded by the Australian Research Council (ARC) and Dyno Nobel Asia Pacific, with grants of computing time from the National Computational Infrastructure (NCI) and the Pawsey Supercomputing Centre in Perth, Australia. I.O. thanks Murdoch University for the award of a postgraduate.

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∫0 α exp(− RTα )dT exp − RTα

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Figure 9 provides validation of the kinetic evaluations. At moderate temperatures, our calculated Eα values predict well (according to eq 7) the conversion−time required to reach a target conversion during the isothermal operation. In practical applications, strict isothermal processing of PE would not be possible, as heating and cooling do not occur at infinite rate. In such situations, for process design under nonisothermal conditions, one can rely on the isoconversional data presented in SI Tables S4 and S5. The fundamental kinetic and conversion measurements revealed in the current contribution make it possible to optimize practical systems for removal of NOx by pyrolysates from neat and recycled PE. In conclusion, the pyrolysates of PE reduced NOx content by about 80% with PE decomposing under the nonisothermal heating conditions. The maximum NOx reduction efficiency of recycled PE remains approximately the same relatively to that of neat samples. The injection of waste PE into a reburning zone of combustors (as well as into bulk ammonium nitratebased explosive formulations7−9,68) may significantly reduce the emission of NOx. Reburning of NOx in practical combustors usually occurs in a temperature window between 1000 and 1200 °C. Therefore, a change of the reaction mechanism may occur at those high temperatures. A subsequent study will address the high-temperature isothermal and flash-particle heating conditions in drop-tube experiments. This will reveal possible effects of the presence of oxygen and mass transfer on the reaction of NOx with PE pyrolysates under realistic reburning temperatures (i.e., 1000−1200 °C). G


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Thermal Reduction of NOx with Recycled Plastics - ACS Publications

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