Two-Step Thermochemical Process for Adding Value to Used Railroad

Research Article pubs.acs.org/journal/ascecg

Two-Step Thermochemical Process for Adding Value to Used Railroad Wood Ties and Reducing Environmental Impacts Pyoungchung Kim,*,† Adam Taylor,† Jeff Lloyd,‡ Jae-Woo Kim,‡ Nourredine Abdoulmoumine,§ and Nicole Labbé*,† †

Center for Renewable Carbon, The University of Tennessee, 2506 Jacob Drive, Knoxville, Tennessee 37996, United States Nisus Corp., 100 Nisus Drive, Rockford, Tennessee 37853, United States § Biosystems Engineering & Soil Science, The University of Tennessee, 2506 E.J. Chapman Drive, Knoxville, Tennessee 37996, United States ‡

S Supporting Information *

ABSTRACT: A two-step thermochemical process combining a thermal desorption at 250−300 °C and a pyrolysis at 500 °C of used creosotetreated wooden railroad ties was carried out to recover preservative and produce a high quality bio-oil and biochar. Under optimal temperature between 280 and 300 °C, high preservative removal efficiency (70− 74%) was achieved with a high proportion of polycyclic aromatic hydrocarbons (PAHs, 80−82%) and a large portion of the original wood mass (67−70%) was retained. This thermally treated biomass had higher heating value (HHV; 19.9−20 MJ/kg) than the starting material. The physical properties of the preservative, such as viscosity and density, and its toxic threshold against a common decay basidiomycete fungus were similar to those of commercially available P2-creosote. Pyrolysis of the thermally treated ties produced bio-oils with lower water content and total acid numbers, and a higher amount of ligninderived compounds than that of untreated ties. Biochars derived from the thermally treated ties contained higher carbon content and lower amount of PAHs than biochars from untreated ties. KEYWORDS: Bio-oil, Biochar, Creosote, Pyrolysis, Polycyclic aromatic hydrocarbons, Thermal desorption, Thermochemical process, Torrefaction, Used railroad ties

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hydrocarbons (PAHs).4 Each wood tie is impregnated with approximately 96−128 kg/m3 creosote. However, during the service life of the tie, about 35% of the original creosote is lost due to volatilization and biological and photochemical degradation.5 The released creosote contaminates soil and groundwater and takes a long time, months or years, to degrade or oxidize so that the carbon fraction is eventually converted to greenhouse gases such as carbon dioxide and methane.6 A lot of effort has been devoted to re-evaluate disposal practices of used wood ties through landfilling, reuse, extraction/detoxification, or thermochemical processes such as pyrolysis, gasification, and combustion.7 Of the disposal practices, solvent extraction and biological detoxification processes remove preservative from treated-wood,8,9 but the processes are extremely expensive and incomplete to remove preservative. Used wood ties could also be an important source for fast pyrolysis.10 Fast pyrolysis of biomass such as wood or organic waste at 450−600 °C generates bio-oil, biochar, and

INTRODUCTION In the U.S., approximately 20−22 million wood ties are replaced annually at the end of their service lives.1 Until recently, the majority of these used wood ties were burned in cogeneration facilities to produce heat and/or electricity, with the remainder being recycled as landscaping timbers or disposed of in landfills.2 However, recent regulatory changes by the United States Environmental Protection Agency (U.S. EPA) under the Non Hazardous Secondary Material Rule (NHSMR) have severely restricted the types of combustion units that can be used.3 These changes have led to many more ties being landfilled. Wood preservation is typically carried out by impregnating wood with preservative chemicals under high pressure to provide long-term resistance to biodegradation. This process is still used as the best option for production of preservative treated wood as a highly functional, cost-competitive, renewable resource.2 Preservative such as creosote and copper naphthenate are used to treat approximately 95% of all wood ties produced in the North America.2 Creosote is a complex mixture of organic compounds obtained by bituminous coal pyrolysis and comprises approximately 85% polycyclic aromatic © 2017 American Chemical Society

Received: August 3, 2017 Revised: August 30, 2017 Published: September 5, 2017 9485

DOI: 10.1021/acssuschemeng.7b02666 ACS Sustainable Chem. Eng. 2017, 5, 9485−9493

Research Article

ACS Sustainable Chemistry & Engineering

(nitrogen gas, 20 L/min) was introduced into the front of the auger reactor and moved with the evolved vapors to a particle chamber (20 cm diameter and 100 cm length) where fine particles in the vapor precipitate. The vapor from the particle chamber was condensed by three condensers (10 cm diameter and 200 cm length). The surface temperature of the condensers was maintained between 10 and 15 °C using a circulating water-cooling system (7 L/min, Chiller, Polyscience Inc.). All liquid fractions were collected into glass bottles connected to the bottom of the condensers, immediately combined, and mixed for homogeneity. The liquid fraction was gravimetrically separated into two phasesa brown colored aqueous phase above a black colored organic phase (Supporting Information Figure S1)and stored in a freezer (−20 °C) until characterization. The solid fraction (thermally treated wood) was collected into the solid collector. Subsequently, the thermally treated biomass was loaded at the rate of 10 kg/h into the auger reactor and pyrolyzed at 500 °C for 70 s. Bio-oils and biochars were collected in the condensers and biochar collector, respectively. The experiment was performed in triplicate. Characterization of the Solid and Liquid Fractions. Solid Fraction. The moisture content of the thermally treated solid (after thermal desorption) and biochar (after pyrolysis) fractions was measured by the Karl Fischer titration method.16 A thermogravimetric analyzer (TGA) was used to evaluate the decomposition of the thermally treated wood samples, as a function of temperature from 30 to 800 °C with a heating rate of 10 °C/min under 20 mL/min of nitrogen gas flow. Proximate analysis, including ash content, volatile matter, and fixed carbon of the samples, was performed according to ASTM D176284.19 Ultimate analysis, including carbon, hydrogen, and nitrogen, was carried out using a CHN analyzer (2400 Series II CHNS/O, PerkinElmer). The higher heating values (HHV) of the samples were calculated according to the method proposed by Channiwala and Parikh.20 Energy yield (HHV retained %), defined by the energy content ratio between thermally treated solid and the untreated wood tie sample, was calculated by the multiplication of the solid yield and the enhancement factor of HHV.21 To quantify the amount of creosote remaining in the wood and biochar after thermal desorption and pyrolysis, respectively, the thermally treated solid and biochar fractions were extracted with dichloromethane (DCM) using an accelerated solvent extractor (ASE 350, Dionex Corp.) at 100 °C and 9 cycles.16 After extraction, the liquid extracts were placed in a separatory funnel and adjusted to a pH higher than 12 with sodium hydroxide (NaOH). The samples were extracted three times using DCM, resulting in the transfer of nonpolar creosote compounds (PAHs) into the DCM phase. The quantity of creosote in DCM was determined gravimetrically after evaporating DCM using a rotary evaporator (IKA) and vacuum drying at 40 °C. Condensed Liquid Fractions. The properties of the condensed liquid fractions (aqueous and organic phases) from desorption and bio-oil from pyrolysis were characterized in triplicate and included pH, water content, density, viscosity, and total acid number (TAN). The pH was measured using a pH meter after mixing the liquid sample (1.0 g) with water (50 mL). The water content was measured using a Karl Fischer titration (Metrohm 787 KF Titrino) according to the American Society for Testing and Material (ASTM D4377) standard.22 The total acid number was determined by titrating the bio-oil (1.0 g) in a mixture of water, isopropyl alcohol, and toluene (1:99:100 v/v) with 0.1 M KOH solution to an end-point of pH 11 according to ASTM D664.23 The chemical composition of the organic fraction from desorption and bio-oils from pyrolysis was determined by gas chromatography/mass spectrometry (GC/MS, PerkinElmer Clarus 680 Gas Chromatograph coupled with a Clarus SQ 8C Mass Spectrometer). A 1.0 μL-aliquot (10 mg liquid in 10 mL acetone) was injected into the GC system. The compounds were then separated by a DB-1701 capillary column (60 m length × 0.25 mm ID × 0.25 μm film thickness, Agilent) in the temperature range of 50−280 °C at 5 °C/min. The separated compounds were analyzed using a MS setup at 270 °C source temperature and 70 eV electron ionization. The generated chromatographic peaks were identified by comparison with the National Institute of Standards and Technology (NIST) mass spectral data library of fragmentation patterns. The kinematic viscosity

noncondensable gases. Typically, bio-oil is acidic (pH 2.5−3.8) with high oxygen content (35−40%), high water content (15− 35 wt %), high viscosity, and low energy density (16−18 MJ/ kg).11 Additionally, bio-oil is immiscible with hydrocarbon fuels and unstable during storage. Biochar can be used as a soil amendment or as an alternative to coal for gasification and cogeneration to produce combined heat and power.12 Torrefaction, a mild thermochemical pretreatment that is carried out at lower temperatures (200−300 °C) has been employed to upgrade biomass into a high quality feedstock for subsequent thermochemical conversion processes such as fast pyrolysis or gasification.13 It produces an improved feedstock with higher energy density, hydrophobicity, grindability, lower moisture content, and more uniform properties. When torrefaction is coupled to fast pyrolysis, it improves the biooil fuel quality by reducing its acidity, lowering its water content, and generating more phenolic compounds.14 A few research groups have produced bio-oils from used creosotetreated wood ties via pyrolysis10 and tested the bio-oil as a preservative for wood protection.15 However, the generated bio-oils containing creosote and wood-decomposed components are difficult to separate or are a less effective non-EPA registered preservative system than virgin creosote. In order to recover preservative from used wood ties, our previous work investigated the potential of thermal treatment in the temperature range of 250−350 °C via a microthermogravimetric analyzer and a bench-scale batch reactor.16,17 We concluded that a thermal treatment between 275 to 300 °C was optimal to recover creosote. However, our previous studies focused just on the thermal desorption of the preservative, not on the pyrolysis of the resulting thermally treated ties and the impact of the thermal desorption on the bio-oil and biochar properties. Moreover, the toxicity of the recovered preservative was not evaluated. In this study, we investigated the entire twostep thermochemical process, which combines a thermal desorption for preservative recovery and a pyrolysis for pyrolytic products via a semipilot scale continuous auger reactor system. The objective of this study was to demonstrate the scalability of a two-step thermochemical process and importantly quantify and assess the quality of the recovered preservatives, bio-oils, and biochars for adding value to used railroad wood ties and reducing environmental impacts.

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EXPERIMENTAL SECTION

Materials. Used creosote-treated railroad ties were obtained from National Salvage & Service Corporation (Bloomington, IN, USA). The ties were ground into less than 4 mm particle size using a knife mill prior to characterization and utilization in our continuous auger system. Two-Step Thermochemical Treatment Using a Continuous Auger System. A semipilot scale continuous auger reactor equipped with a feeding system, a rectangular auger reactor, a vapor condensation section, and a solid collector was used to thermally treat the used railroad tie materials to recover the creosote remaining after 30 years of service and subsequently produce pyrolytic products, bio-oil, and biochar, from the thermally treated material. A detailed description of the auger reactor system is provided elsewhere.18 In brief, approximately 10 kg of the creosote-treated wood was transferred from the feeding hopper to the auger reactor by a single auger with a feeding rate of approximately 2.5 kg/h. The auger reactor (10 cm width × 10 cm height × 250 cm length) contained internal dual augers. The auger speed controlled the solid residence time at approximately 15 min. The heated zone comprised a 200 cm long electrical resistance furnace with the reaction temperature at 250, 280, or 300 °C to volatilize the creosote from the biomass. A sweeping gas 9486


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ACS Sustainable Chemistry & Engineering Table 1. Thermal Desorption Products Yield of Used Railroad Ties product (wt %) liquid thermal desorption temp (°C) 250 280 300

totala c

27.6 (0.6) 35.1 (1.4) 38.7 (0.6)

aqueous phase

organic phase

solid

NCGb

25.7 (0.2) 32.0 (0.1) 34.1 (0.5)

1.9 (0.2) 3.1 (0.1) 4.6 (0.5)

64.5 (2.8) 56.5 (0.4) 53.7 (1.5)

7.8 (2.3) 8.5 (1.2) 7.6 (0.9)

a Total is the sum of the aqueous and organic (creosote) phase. bNCG represents noncondensable gases and was calculated by difference (100 − liquid − solid). cAverage and standard deviation in parentheses (n = 3).

of the organic phase and bio-oils was measured at 40 °C with serialized Schott Ubbelohde capillary viscometers according to the ASTM D445 standard.19 Thermally decomposed wood compounds in the aqueous phase produced from the thermal desorption were quantified by highperformance liquid chromatography (HPLC, PerkinElmer 200 series).24 The analysis was conducted at 55 °C using a Bio-Rad Aminex HPX-87H column (300 × 7.8 mm, 9 μm particle size) with a refractive index (RI) detector. The injected volume was 20 μL and the mobile phase was 5 mM sulfuric acid in deionized water at a flow rate of 0.6 mL/min. The molecular weight distribution (MWD) of the organic phase from desorption was determined by gel permeation chromatography (EcoSEC GPC, TOSOH Bioscience LLC) equipped with a TSKgel SupermultiporeHZ-M column (4.6 × 150 mm; 4 μm) and an RI detector. The sample (2 mg) was dissolved in 10 mL of tetrahydrofuran (THF), then filtered through a 0.2-μm poresize microfilter. The filtered sample was analyzed at a flow rate of 0.35 mL/ min of THF at 40 °C. The elution time was converted to molecular weight by calibration with polystyrene standards. The generated organic fraction from the desorption treatment and the bio-oil were solvent-fractionated into four groups: PAHs, high molecular weight lignin-derived phenolic compounds (labeled as phenolics), sugarcontaining materials including levoglucosan, dimer, and trimer anhydro-polysaccharides (labeled as sugars), and ether-soluble compounds including aldehydes, ketones, volatile acids, and lignin monomers (labeled as light oxygenates) for the distribution of PAHs and wood-derived compounds.25 The solvent fractionation procedure is detailed in previous work.16 The toxicity threshold of the recovered creosote against a common reference wood decay fungus was determined according to the AWPA standard E10.26 Briefly, southern pine (Pinus spp.) sapwood cubes were treated with either recovered creosote at 280 °C or commercially available creosote. Both creosotes were diluted with toluene and then treated to the target retentions (between 0 and 27 kg/m3) in the wood blocks including no-creosote controls. After treatment and drying to evaporate the toluene, the blocks were exposed to the brown rot fungus Postia placenta in bottles filled with moist soil for 12 weeks. The weight loss percentage after the fungal exposure was calculated.

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Figure 1. TG thermograms (a) and derivative of TG (b) curve of untreated and thermally treated wood tie samples at 250, 280, and 300 °C.

finally slowly increased to 800 °C. The gradual increase of weight loss between 30 and 200 °C was mainly due to the desorption of water and creosote present in the wood ties. The drastic weight loss between 200 and 410 °C corresponded to the desorption of creosote compounds with different boiling points ranging from 200 to 400 °C as well as the initial decomposition of the wood matrix. The thermograms of the thermally treated wood samples showed no weight loss between 30 to 250 °C, but a drastic weight loss between 250 and 400 °C. The derivative of the thermograms (DTG) exhibited a slight reduction of the shoulder peak at 315 °C for the sample treated at 250 °C compared to the untreated sample, presumably, because hemicellulose starts to thermally decompose at 250 °C, but cellulose and lignin are only slightly affected.27 The samples treated at 280 and 300 °C possessed a single major peak at 305 °C without the shoulder peak. The depletion of the shoulder peak indicated that hemicellulose was degraded during the thermal desorption of the preservative at 280−300 °C with slight degradation of cellulose and lignin. These various weight losses are explained by dehydration, fragmentation, and depolymerization of cellulose, hemicellulose, and lignin in the wood matrix28 and the desorption of creosote. Properties of Thermally Treated Wood Ties. After 30 years of service, used railroad ties contained approximately 7.0

RESULTS AND DISCUSSION

Thermal Desorption of Creosote. As desorption temperature progressed from 250 to 300 °C, condensed liquid yield increased from 27.6 to 38.7 wt % while solid yield decreased from 64.5 to 53.7 wt % (Table 1); this represented a solid weight loss of 19 to 33 wt % (dry basis). The yields of both organic (preservative) and aqueous phases in the condensed liquid fraction increased from 1.9 to 4.6 wt % and from 25.7 to 34.1 wt %, respectively. To understand the characteristics of thermal desorption of creosote and degradation of the wood matrix, we investigated the thermally treated wood tie samples using thermogravimetric analysis (TGA; Figure 1). In general, creosote compounds consist of aromatic rings ranging from 1 to 5 and have boiling points from 218 to 495 °C and vapor pressures from 12.3 to 10−6 Pa at 25 °C.6 The weight loss of the untreated wood in the TG thermograms gradually increased from 30 to 200 °C, then drastically increased to 410 °C, and 9487


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samples was determined from the Py-GC/MS pyrograms (Figure 2b). An increase of desorption temperature reduced the proportion of 2- and 3-aromatic ring compounds such as naphthalene, acenaphthylene, acenaphthene, fluorene, anthracene, phenanthrene, and fluoranthene with boiling points ranging from 218 to 375 °C and vapor pressures ranging from 12.3 to 1.2 × 10−3 Pa at 25 °C and increased that of 4- and 5aromatic ring compounds such as pyrene, benzanthrene, and benzo(e)pyrene with boiling points from 393 to 495 °C and vapor pressures from 6.0 × 10−4 to 1.6 × 10−6 Pa (Figure 2b and Supporting Information Table S1). These results demonstrated that when creosote-treated wood materials were exposed to an elevated temperature at 280−300 °C, 70−74% of the PAH compounds were desorbed while PAH components with relatively higher boiling point and lower vapor pressure remained in the thermally treated wood. Moreover, by thermally treating the wood ties at temperatures ranging from 250 to 300 °C, the volatile matter was reduced from 82.1 to 70.2% and the fixed carbon content increased from 15.4 to 25.5% (Table 2). In addition, performing the PAH desorption at high temperature augmented carbon content from 47.0 to 53.0% and diminished oxygen content from 44.4 to 37.1% and hydrogen content from 5.9 to 5.3% of the ties. These results influenced the changes in energy value of the thermally treated wood; the higher heating values (HHVs) increased from 18.7 to 20.8 MJ/kg whereas energy yield decreased from 100 to 77.6%. The atomic O/C and H/C ratios of the treated ties at 280 and 300 °C ranged from 0.52 to 0.55 and from 1.2 to 1.23, respectively (Figure 2c), which were values comparable to that of torrefied biomass.13 The van Krevelen diagram of these materials showed a strong linear correlation between the atomic O/C and H/C ratios upon thermal treatment (Figure 2c), proving that the thermal treatment shifted the elemental ratios of wood ties toward that of coal.29 These findings suggested that thermally treated wood ties at 280−300 °C could be suitable for downstream thermochemical processes such as pyrolysis and gasification to produce biofuel, heating, and electricity. A higher PAH level could be recovered with desorption temperature higher than 300 °C; however, a trade-off between PAHs recovery and energy yield (HHV retained) loss through degradation of the biomass constituents16 will need to be considered. Properties of Desorbed Liquid Products. Aqueous Fraction. As the thermal desorption temperature increased, the water content in the aqueous fraction decreased from 94.7 to 88.2%, the pH dropped from 3.2 to 2.6, and the total acid number (TAN) increased from 19.9 to 62.8 mg KOH/g (Table 3). As mentioned earlier, desorption temperatures between 280 and 300 °C induced the partial degradation of the solid matrix

wt % of PAHs (Figure 2a). Residual amount of creosote remained in the thermally treated wood tie samples; the 250

Figure 2. Distribution of creosote (PAHs) in the thermally treated wood ties and van Krevelen diagram of solid fraction. (a) Creosote (PAHs) amount remained in the solid fraction. (b) Distribution of PAH aromatic rings in creosote by Py-GC/MS. (c) van Krevelen diagram of solid fraction.

°C-treated sample contained 51% (3.6 wt %) whereas the 280 and 300 °C-treated samples had 30% (2.1 wt %) and 26% (1.8 wt %), respectively. The chromatographic peak area percentage of PAH compounds in the untreated and thermally treated

Table 2. Proximate and Elemental Analysis and HHV of Thermally Treated Wood Tie Samples proximate analysis (%) desorption temp (°C) untreated 250 280 300

moisturea 20.0 2.7 2.5 2.7

(0.1) (0.1) (0.2) (0.1)

volatile matter 82.1 79.8 74.4 70.2

(0.8) (1.8) (1.3) (1.2)

fixed carbon 15.4 16.1 22.1 25.5

(0.7) (0.9) (1.4) (1.2)

ultimate analysis (%) ash 2.5 3.4 3.6 4.3

(0.2) (0.1) (0.1) (0.1)

C 47.0 50.1 52.4 53.0

(0.7) (0.4) (0.7) (0.3)

H 5.9 5.9 5.4 5.3

(0.1) (0.0) (0.1) (0.0)

N 0.2 0.2 0.2 0.3

(0.0) (0.0) (0.0) (0.0)

O 44.4 40.6 38.4 37.1

(0.8) (0.4) (0.7) (0.4)

HHVb (MJ/ kg) 18.7 19.9 20.6 20.8

(0.4) (0.2) (0.4) (0.2)

energy yieldc (%) 100 89.0 (0.9) 80.8 (1.5) 77.6 (0.7)

Moisture content was measured by Karl Fischer titration method. bHHV (MJ/kg) calculated by HHV = 0.3491 × carbon + 1.1783 × hydrogen − 0.1034 × oxygen − 0.015 × nitrogen − 0.0211 × ash. cEnergy yield was calculated from the weight loss and HHVs in the untreated and thermally treated wood samples. a

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ACS Sustainable Chemistry & Engineering Table 3. Physical Properties of the Desorbed Liquid (Organic and Aqueous) Fractions aqueous phase

organic phase molecular distribution

temp (°C) 250 280 300 P2f

pH

water (%)

TANa

pH

g

94.7 (1.7) 91.8 (0.8) 88.2 (0.4)

19.9 (0.9) 53.5 (2.3) 62.8 (0.6)

4.2 (0.2) 3.7 (0.1) 3.5 (0.1) NDh

3.2 (0.1) 2.7 (0.0) 2.6 (0.0)

water (%) 13.8 7.9 6.7 4.8

(2.1) (3.4) (0.8) (0.1)

TAN 12.5 (1.3) 24.7 (3.2) 28.1 (0.9) ND

viscosity (mm2/s)b

density (g/cm3)

8.8 12.3 14.5 14.9

1.2 1.2 1.2 1.2

(0.1) (0.2) (0.7) (0.4)

Mnc

(0.0) (0.0) (0.0) (0.0)

100 90 74 128

(3) (7) (4) (3)

Mwd 299 (10) 275 (10) 240 (4) 382 (11)

Mw/Mne 3.0 3.1 3.2 3.0

(0.2) (0.1) (0.1) (0.1)

TAN represents total acid number (mg KOH/g). bViscosity was measured at 40 °C. cMn represents the number-average molecular weight. dMw represents the weight-average molecular weight. eMw/Mn represents the polydispersity. fP2 is the commercially available P2-creosote produced from coal tar. gAverage and standard deviation in parentheses (n = 3). hND represents nondetection. a

and cellulose to a certain extent.30 These results agreed well with our thermogravimetric findings (Figure 1), where the loss of a shoulder peak was attributed to the thermal decomposition of hemicellulose and cellulose that takes place between 280 and 300 °C. Organic Fraction. The black colored organic (preservative) fraction contained a small amount of water (6.7−13.8 wt %) and had low TAN values (12.5−28.1 mg KOH/g), leading to higher pH (3.5−4.2), viscosity (8.8−14.5 mm2/s), and density (1.2 g/cm3) when compared to the aqueous fraction (Table 3). The recovered organic fraction had viscosity and density similar to commercially available P2-creosote whereas the aqueous fraction contained mostly water-soluble, wood-decomposed compounds depending on density and polarity of the compounds (Table 3). A total of seventy-six chemical compounds in the organic phase were identified using GC/MS and were solventfractionated into light oxygenates, sugars, phenolics, and PAHs-derivatives (Figure 3b and Supporting Information Table S2). These recovered creosote components were mostly polycyclic aromatic hydrocarbons (PAHs), such as naphthalene, acenaphthylene, acenaphthene, phenanthrene, anthracene, pyrene, and other derivatives. Phenolic derivatives (8 wt %), sugar derivatives (14 wt %, furfural, levoglucosan), and light oxygenated compounds (0.8 wt %, acetic acid) were found in the organic fraction of the sample treated at 250 °C. As temperature rose to 280 and 300 °C, PAHs increased to 80 and 82 wt % and phenolic compounds increased to 10 and 13 wt % while sugars decreased to 8 and 4%, respectively (Figure 3b). The organic fraction included mainly PAHs and phenolic compounds (Supporting Information Table S2). A small amount of sugars and light oxygenated compounds was also found in the organic phase, which was probably dissolved in water (6.7−13.8 wt %) in that phase (Table 3). The molecular weight analysis of the organic fraction showed a decrease in the weight average (Mw) from 299 to 240 g/mol and number average (Mn) from 100 to 74 with temperature. These values were only slightly lower than that of the commercially available P2-creosote. The molecular weight distribution (Figure 4) also demonstrated that increasing temperature generated lower molecular weight compounds, due to a gradual decrease in the amount of anhydrosugars and increase of monomeric phenolic compounds decomposed from lignin in the wood matrix (Figure 3b). To determine the distribution of PAH compounds in the organic fraction, a total of 65 PAH derivatives were identified by GC/MS and categorized based on aromatic ring numbers (Supporting Information Table S2). The GC/MS peak area percentage of PAH compounds (Figure 3c) showed that the

and produced compounds derived from hemicellulose and cellulose, including acetic acid, acetol, furfural, hydroxymethylfurfural (HMF), glycerol, and levoglucosan (Figure 3a). A significant increase of acetic acid in the 280 and 300 °Caqueous fractions can be explained by the degradation of most of the hemicellulose with thermally unstable O-acetyl groups

Figure 3. Chemical compounds in the desorbed liquid fraction. (a) Carbohydrate-derived compounds in the aqueous fraction by HPLC. (b) Compounds present in the organic phase by solvent fractionation. (c) Distribution of chromatographic peak area (%) of PAH aromatic rings in the organic fraction by GC/MS. 9489


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like that of a conventional creosote sample, and to a published reference value (9.6 kg/m3)33 for the fungus examined (Figure 5). These findings indicated that the efficacy of the recovered

Figure 5. Efficacy of recovered preservative at 280 °C in soil-block test against the fungus postia placenta.

preservative with a low impurity of wood-derivatives was very similar to that of the commercial creosote. The findings also suggested that the recovered preservative had the potential to be a useful supplement or substitute to creosote; however, more testing would be necessary to support these preliminary findings, including investigating other fungi and wood species. Pyrolysis of Thermally Treated Ties. Pyrolytic Products. In this section, we used the thermally treated wood materials as a feedstock for pyrolysis and evaluated the quality of the biooils and biochars generated at 500 °C. Pyrolysis of untreated wood tie samples produced 61 wt % of bio-oil and 19 wt % of biochar (Figure 6a). However, with an elevated thermal desorption temperature (250 to 300 °C), the bio-oil yield noticeably dropped, ranging from 42 to 35% while the biochar yield increased from 25 to 35 wt %. The reduction of bio-oil yield can be explained by the decomposition of thermally unstable hemicellulose and cellulose as well as the desorption of wood preservatives during the initial thermal treatment between 250 and 300 °C. This resulted in an increase in the relative content of lignin in the thermally treated wood materials producing a high lignin-derived bio-oil (Figure 6b). Bio-oil Properties. The properties of the bio-oils produced from the untreated and thermally treated ties were presented in Table 4. The water content produced from the untreated wood ties was 60%, which mostly arose from the moisture naturally present (∼20 wt %) in the wood tie and dehydration reaction of cellulose and hemicellulose during pyrolysis. The thermally treated wood ties with increasing temperatures from 250 to 300 °C generated bio-oil with lower water content from 36 to 32 wt % due to drying and dehydration reactions during thermal desorption, which induced an increase in the kinematic viscosity. In addition, a thermal desorption step with an elevated severity reduced the total acid number (TAN) from 127 to 113 mg KOH/g. The TAN value of the bio-oil produced from the untreated sample was 78 mg KOH/g, likely due to dilution by the high water content of the bio-oil. Lowering TAN values may be due to the reduction of hemicellulose portion during the thermal desorption of the creosote, decomposing the hemicellulose into mainly acetic acid and other acidic compounds. The bio-oil compounds derived from the thermally untreated and treated samples were identified using GC/MS (Supporting

Figure 4. Molecular weight distribution of the desorbed organic fraction: (a) differential and (b) integral distributions.

organic fraction for thermal treatment at 250 °C had a higher peak area proportion of mainly 2- (45%), 3- (43%), and 4aromatic rings (11%), as well as traceable amount of 1- and 5aromatic ring creosote compounds (less than 1%). As the temperature was raised to 300 °C, the percentage of 2-aromatic ring compounds such as, naphthalene, biphenyl, acenaphthylene, acenaphthene, fluorene, and derivatives, decreased to 38%, while that of 3- and 4-aromatic ring compounds such as, anthracene, phenanthrene, fluoranthene, pyrene, benzanthrene, and their derivatives, increased to 46% and 15%, respectively. These results indicated that, as temperature increased, the proportion of PAH compounds with low boiling point and high vapor pressure decreased. The PAH compounds recovered from the used wood ties also had a lower proportion of 2aromatic ring compounds and higher proportion of 3-, 4-, and 5-aromatic ring compounds than the commercially tested creosote sample (Figure 3c and Supporting Information Table S3). These findings confirmed that a larger portion of PAH compounds with lower molecular weights and higher vapor pressures was removed during service life due to degradation, evaporation, or leaching.5,31,32 Potential Reuse of Thermally Recovered Creosote. If the recovered creosote is to be reused as a wood preservative, its efficacy must be verified to ensure adequate performance. A complete investigation of the potential for recovered preservative to supplement or replace commercially available creosote was beyond the scope of this study, but a preliminary test was conducted to evaluate the toxicity threshold of the recovered creosote at 280 °C against a common reference wood decay fungus using a standard, lab-scale test method based on AWPA standard E10.26 The results demonstrated that most of sapwood cubes with higher than about 9 kg/m3 of the recovered creosote had less than 10% weight loss, which is comparable to that of the commercial creosote. Thus, the toxicity threshold for the recovered preservative appeared to be 9490


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the properties of bio-oils produced from torrefied biomass reported in the literature.14 Biochar Properties. Biochars generated from the pyrolysis of untreated wood samples contained 35% of volatile matter and 56% of fixed carbon content (Table 5). Biochars produced from the solid samples thermally treated with an elevated temperature from 250 to 300 °C had a reduction in volatile matter content from 27 and 22% and an increase in the fixed carbon from 60 and 63%. These results demonstrated that thermally treated solids contained lower volatile matter and higher fixed carbon content than untreated wood tie sample. In addition, the higher heating values (HHVs) of biochars produced from thermally treated solids ranged between 25.0 and 26.4 MJ/kg, which was higher than that of biochar derived from an untreated wood sample. The van Krevelen diagram (Figure 7a) showed that the atomic O/C and H/C ratios of biochars derived from the thermally treated solids with rising temperature from 250 and 300 °C diminished from 0.22 to 0.16 and increased from 0.51 to 0.54, respectively, by decarboxylation and demethylation reactions.34 In addition, ash content including mineral compounds such as K, Mg, Ca, and Fe, in the biochars of the thermally treated ties gradually increased with temperature. The residual amount of PAHs remaining in the biochars derived from untreated wood sample was 1.2 wt % accounting for 83% removal of PAHs present in the original used railroad wood tie (Figure 7b). Biochars produced from thermally treated solids with an elevated temperature from 280 to 300 °C had very low PAH content from 0.5 to 0.3 wt % with 93−96% removal efficiency. These findings demonstrated that with an initial desorption of a large amount of valuable preservatives from used wood ties, volatile organic hazardous air pollutants were dramatically removed from the biochars, which could then become a potential boiler fuel with high heating values leading to reduction of air pollution emission in the facilities when the biochars are used in common biomass boilers.

Figure 6. Yield of pyrolytic products (i.e., bio-oil, biochar, and noncondensable gases) after thermal treatment of ties (a) and compounds in bio-oil (b) produced by solvent fractionation of bio-oil. 250/500, 280/500, and 300/500 correspond to the desorption and pyrolysis temperatures in degrees Celcius, respectively.

Information Table S4) and solvent-fractionated into light oxygenates, sugars, phenolics, and PAH-derivatives groups (Figure 6b). The bio-oil derived from untreated sample contained phenolics (13 wt %), sugar derivatives (54 wt %, mainly levoglucosan), light oxygenated compounds (7 wt %, acids, aldehyde, esters, alcohols, etc.), and PAH derivatives (28 wt %). Pyrolysis of ties treated at temperatures increasing from 250 to 300 °C generated bio-oils with slightly lower sugar derivative yield from 53 to 51 wt % and higher phenolic compounds from 20 to 33 wt % due to decreasing hemicellulose and cellulose portions and relatively increasing lignin content in the thermally treated wood samples. The light oxygenated fraction also decreased from 6 to 4 wt %. The PAH fraction in the bio-oils also noticeably dropped from 21 to 11 wt %. Therefore, pyrolysis of thermally treated wood samples affected the distribution of bio-oil compounds and produced a high quality of bio-oil with low water content, acidity, and high pyrolytic lignin-derived compounds, which was consistent with

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CONCLUSIONS A two-step thermochemical process using a semipilot scale continuous auger reactor was used to recover high quantity PAH compounds with some minor wood-decomposed compounds from used creosote-treated wood and subsequently produce a better quality of bio-oil and biochar than that of untreated ties. The recovered preservative had physical and chemical properties similar to commercially available P2creosote. Preliminary toxic threshold testing suggested the potential to reuse the recovered preservative as a wood preservative as it has similar efficacy. The thermally treated wood, having low residual PAHs, high HHV, and energy yield, was pyrolyzed and produced bio-oil with lower water content and acidic compounds and higher lignin-derived components than bio-oil produced from untreated wood ties. Biochar

Table 4. Physical Properties of Bio-Oils Produced from Thermally Treated Wood Materials sample c

250/500 280/500 300/500 untreated/500

pH 2.8 2.8 2.8 2.9

(0.0) (0.0) (0.0) (0.0)

water (%) d

35.6 32.9 32.0 60.4

TANa (mg KOH/g)

(2.0) (1.7) (0.8) (4.7)

127 (5) 121(3) 113 (1) 78 (2)

density (g/mL) 1.2 1.2 1.2 1.1

(0.0) (0.0) (0.0) (0.0)

viscosityb (mm2/s) 8 12 14 1

(1) (1) (1) (0)

TAN represents total acid number. bViscosity was measured at 40 °C. c250/500, 280/500, and 300/500 correspond to the desorption and pyrolysis temperatures in degress Celcius, respectively. dAverage and standard deviation in parentheses (n = 3). a

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Table 5. Proximate and Elemental Analysis, HHV, and Inorganic Composition of Biochars Produced from Thermally Treated Wood Tie Samples proximate analysis (%) sample untreated wood 250/500c 280/500 300/500 untreated/500 sample untreated wood 250/500 280/500 300/500 untreated/500

moisturea 20.0 4.6 4.3 3.8 4.6

(0.1) (0.4) (1.2) (0.5) (0.1)

Ca 2139 7464 7790 7966 7241

(116) (502) (446) (329) (559)

volatile matter 82.1 26.6 23.3 21.9 34.8

(0.8) (1.1) (2.3) (2.4) (2.1) Fe

4355 (497) 15565 (1335) 16498 (1355) 16359 (1338 11697 (933)

ultimate analysis (%)

fixed carbon 15.4 59.7 62.5 63.0 55.8

(0.7) (1.9) (1.2) (1.7) (1.9)

ash 2.5 (0.2) 12.4 (0.8) 12.9 (1.0) 13.7 (1.3) 8.3 (0.4) Inorganic

K 4355 14989 16388 16926 11697

C

379 760 881 1055 679

N

47.0 (0.7) 5.9 (0.1) 68.7 (0.7) 2.9 (0.1) 69.8 (0.7) 3.2 (0.0) 68.4 (0.2) 3.1 (0.0) 64.4 (1.1) 5.9 (0.1) Compounds (mg/kg)

Mg (30) (48) (96) (75) (74)

H

(36) (44) (96) (61) (34)

0.2 0.5 0.5 0.5 0.5

HHVb (MJ/kg)

O

(0.0) (0.0) (0.0) (0.0) (0.0)

44.4 20.6 14.4 15.4 25.0

(0.8) (0.8) (0.8) (0.2) (1.3)

18.7 25.0 26.4 25.7 23.5

(0.4) (0.4) (0.4) (0.1) (0.7)

Na

P

S

Si

Mn

334 (82) 1201 (106) 1203 (113) 1322 (47) 911 (46)

50 (6) 175 (16) 215 (15) 264 (36) 164 (27)

691 (8) 675 (56) 712 (93) 763 (56) 666 (75)

6693 (550) 21570 (5109) 23529 (3201) 24679 (1058) 16234 (426)

91 (3) 343 (8) 341 (15) 324 (6) 302 (11)

a Moisture content was measured by Karl Fischer titration method. bHHV (MJ/kg) was calculated by HHV = 0.3491 × carbon + 1.1783 × hydrogen − 0.1034 × oxygen − 0.015 × nitrogen − 0.0211 × ash. c250/500, 280/500, and 300/500 correspond to the desorption and pyrolysis temperatures in degrees Celcius, respectively.

mental impacts by preventing the releasing of creosote in the environment during landfilling.

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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/acssuschemeng.7b02666. Figure S1. Liquid fraction generated using a semipilot scale continuous auger reactor. Table S1. Properties of creosote compounds including molecular weight, boiling point, and vapor pressure. Table S2. Chemical compounds of the organic fraction of creosote-treated wood tie generated via thermal desorption treatment by GC/MS. Table S3. Chemical compounds of commercially available P2-creosote compounds by GC/MS. Table S4. Chemical compounds of bio-oils of creosotetreated wood ties produced via pyrolysis at 500 °C by GC/MS (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: (865) 974-5086. Fax: (865) 946-1109 (P.K.). *E-mail: [email protected]. Phone: (865) 946−1126. Fax: (865) 946−1109 (N.L.). ORCID

Pyoungchung Kim: 0000-0001-6270-0473 Nourredine Abdoulmoumine: 0000-0001-6586-5919 Nicole Labbé: 0000-0002-2117-4259

Figure 7. Van Krevelen diagram of thermally treated ties and biochars (a) and PAHs residue in the biochars produced by pyrolysis (b). 250/ 500, 280/500, and 300/500 correspond to the desorption and pyrolysis temperatures in degrees Celcius, respectively.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the 2014−2015 AgResearch Innovation grant (the University of Tennessee Institute of Agriculture) and USDA Agriculture and Food Research Initiative (AFRI, grant no. 2015-6021-24121) for financial support and Nisus Corporation (Rockford, TN, USA) for providing the ties for this work.

produced from the thermally treated wood materials had higher fixed carbon content, heating values, and low amount of PAHs, which could be a potential source for combustion with reduced air pollution or for improving soil properties and plant growth. We concluded that the proposed two-step thermochemical process could be a commercially viable pathway to add value to readily available used ties and ultimately reduce its environ9492


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(24) Sluiter, A.; Hames, B.; Ruiz, R.; Scarlata, C.; Sluiter, J.; Templeton, D. Determination of sugars, byproducts, and degradation products in liquid fraction process samples: Laboratory analytical procedure (LAP); National Renewable Energy Laboratory, 2008. (25) Sipilä, K.; Kuoppala, E.; Fagernäs, L.; Oasmaa, A. Characterization of biomass-based flash pyrolysis oils. Biomass Bioenergy 1998, 14 (2), 103−113, DOI: 10.1016/S0961-9534(97)10024-1. (26) Standard method to testing wood preservatives by laboratory soilblock cultures; American Wood Preservers Association, 2009; Vol. AWPA-E10. (27) Rousset, P.; Aguiar, C.; Labbe, N.; Commandre, J. M. Enhancing the combustible properties of bamboo by torrefaction. Bioresour. Technol. 2011, 102 (17), 8225−8231. (28) Scheirs, J.; Camino, G.; Tumiatti, W. Overview of water evolution during the thermal degradation of cellulose. Eur. Polym. J. 2001, 37 (5), 933−942. (29) Chew, J. J.; Doshi, V. Recent advances in biomass pretreatment − Torrefaction fundamentals and technology. Renewable Sustainable Energy Rev. 2011, 15 (8), 4212−4222, DOI: 10.1016/ j.rser.2011.09.017. (30) Wang, S. R.; Ru, B.; Lin, H. Z.; Luo, Z. Y. Degradation mechanism of monosaccharides and xylan under pyrolytic conditions with theoretic modeling on the energy profiles. Bioresour. Technol. 2013, 143, 378−383. (31) Kohler, M.; Kunniger, T.; Schmid, P.; Gujer, E.; Crockett, R.; Wolfensberger, M. Inventory and emission factors of creosote, polycyclic aromatic hydrocarbons (PAH), and phenols from railroad ties treated with creosote. Environ. Sci. Technol. 2000, 34 (22), 4766− 4772. (32) Marcotte, S.; Poisson, T.; Portet-Koltalo, F.; Aubrays, M.; Basle, J.; de Bort, M.; Giraud, M.; Hoang, T. N.; Octau, C.; Pasquereau, J.; Blondeel, C. Evaluation of the PAH and water-extractable phenols content in used cross ties from the French rail network. Chemosphere 2014, 111, 1−6. (33) Freeman, M. H.; Nicholas, D. D.; Renz, D.; Buff, R. In PXTS: A metal free oligomer wood preserving system−A summary of data to data; American Wood-Preservers’ Association: Boston, MA, 2003; p 167. (34) Kim, P.; Johnson, A.; Edmunds, C. W.; Radosevich, M.; Vogt, F.; Rials, T. G.; Labbe, N. Surface Functionality and Carbon Structures in Lignocellulosic-Derived Biochars Produced by Fast Pyrolysis. Energy Fuels 2011, 25 (10), 4693−4703.

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Two-Step Thermochemical Process for Adding Value to Used Railroad

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