Characterization analysis of Poplar fluff pyrolysis products. Multi-component kinetic study

Fuel 238 (2019) 111–128

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Characterization analysis of Poplar fluff pyrolysis products. Multi-component kinetic study

T

Bojan Jankovića, , Nebojša Manićb, Vladimir Dodevskic, Jasmina Popovićd, Jelena D. Rusmiroviće, Miloš Tošića ⁎

a

University of Belgrade, Institute of Nuclear Sciences “Vinča”, Department of Physical Chemistry, Mike Petrovića Alasa 12-14, P.O. Box 522, 11001 Belgrade, Serbia University of Belgrade, Faculty of Mechanical Engineering, Fuel and Combustion Laboratory, Kraljice Marije 16, P.O. Box 35, 11120 Belgrade, Serbia c University of Belgrade, Institute of Nuclear Sciences “Vinča”, Laboratory for Materials Sciences, Mike Petrovića Alasa 12-14, P.O. Box 522, 11001 Belgrade, Serbia d University of Belgrade, Faculty of Forestry, Department of Chemical and Mechanical Wood Processing, Kneza Višeslava 1, 11030 Belgrade, Serbia e University of Belgrade, Faculty of Technology and Metallurgy, Innovation Center, Karnegijeva Street 4, 11120 Belgrade, Serbia b

ARTICLE INFO

ABSTRACT

Keywords: Poplar fluff Pyrolysis Characterization of pyrolytic products Graphitization Presence of catalysts Polycyclic aromatic hydrocarbons (PAH’s)

This paper describes the pyrolysis of Poplar fluff (from Populus alba) using on-line apparatus, and carbonization process at 850 °C using the fixed bed reactor. Characteristics of pyrolysis products were examined. Elemental and chemical analyses were shown that Poplar fluff has higher energy content characterized by increased content of fibrous structure (particularly cellulose). Independent parallel reactions model very well describes devolatilization process. It was found that increased amount of extractives can significantly affect on increased release of light gaseous products, but declining hydrocarbons, mostly the alkanes. Liquid product is mainly composed of phenolics, aldehydes, acids, esters and ketones. The carbonization process produces the great abundance of polycyclic aromatic hydrocarbons (PAH’s), where naphthalene is the most abundant. Mechanism for PAH’s formation was suggested. This study represents the first step in a much wider and more comprehensive way in thermal conversion processes of this type of fuel.

1. Introduction Biomass, sun (e.g. photovoltaic solar cells and solar heat collectors), wind (e.g. wind turbines), water (e.g. the hydropower, tidal energy) and geo-thermal resources are all sources of renewable energy, but the biomass is the only renewable resource of carbon for production of chemicals, materials, and fuels. The industrial production of a wide range of chemicals and synthetic polymers heavily relies on fossil resources [1]. The European Union (EU) has already approved the laws for reduction of environmentally abusive materials and started to put greater efforts in finding eco-friendly materials based on the natural resources. Hence, the alternative solutions are sought to develop sustainable polymers from renewable natural resources for decreasing the current dependence on the fossil resources and fixing the production rate of CO2 to its consumption rate [2,3]. Biomass and biomass derived materials have been pointed out to be one of the most promising alternatives [4,5]. These materials are generated from available atmospheric CO2, water and the sunlight through biological photosynthesis. Therefore, biomass has been considered to be the only sustainable source of

⁎

organic carbon in earth and perfect equivalent to petroleum for production of fuels and the fine chemicals with net zero carbon emission [6,7]. In this manner, the lignocellulosic biomass, which is the most abundant and bio-renewable biomass on the Earth, has a critical importance. Many studies have shown that lignocellulosic biomass holds enormous potential for sustainable production of chemicals and fuels. The lignocellulosic biomass has been projected as an abundant carbonneutral renewable source, which can decrease CO2 emissions and atmospheric pollution. Thus, it is a promising alternative to limit crude, which can be utilized to produce bio-fuels, biomolecules, and biomaterials [8–10]. Lignocellulosic biomass can be a generous source for energy, fuels, and chemicals. There are many bioenergy routes which can be used to convert biomass feedstocks into a final energy products. Upgrading technologies for biomass feedstocks (e.g. gasification, pelletisation, torrefication, and pyrolysis) are being developed to convert bulky raw biomass into denser and more practical energy carriers for more efficient transport, storage, and convenient use [11]. The fact that the main biomass pseudo-components (fractions), cellulose, hemicelluloses, and lignin, react differently at the different temperatures to yield different

Corresponding author. E-mail address: [email protected] (B. Janković).

https://doi.org/10.1016/j.fuel.2018.10.064 Received 23 August 2018; Received in revised form 5 October 2018; Accepted 10 October 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.

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the apparent activation energy (kJ mol−1) absolute temperature (K), the gas constant (8.314 J·mol−1·K−1) time (min) reaction order serial number of the pseudo-component serial number of the heating rate apparent activation energy of i-th pseudo-component (kJ mol−1) Ai the pre-exponential factor of i-th pseudo-component (min−1) reaction order related to i-th pseudo-component ni αi conversion degree related to i-th pseudo-component total conversion degree αN N total number of reactions φi contribution of the partial reaction OBF objective function the number of heating rates Nk Nm the number of TGA measurements M the number of the data points max |dα/dt|exp,m the maximum experimental conversion rate value (min−1) G Guaiacyl S Syringyl PSC pseudo-component of biomass QOF quality of the fit (%) Tp the maximum (peak) temperature (oC) ΔσPSC difference in dispersion of deconvoluted peak related to a given pseudo-component (oC) PAH’s polycyclic aromatic hydrocarbons NAP naphthalene NAP-1-m naphthalene, 1-methyl HACA hydrogen abstraction acetylene addition

Nomenclature

Ea T R T N I J Ea,i

AIC Akaike information criteria CO2 carbon dioxide CO carbon monoxide methane CH4 C, H, O, N, S carbon, hydrogen, oxygen, nitrogen, sulphur TGA Thermogravimteric analysis DSC differential scanning calorimetry DTG derivative thermogravimetry STA simultaneous thermal analysis STA-QMS simultaneous thermal analysis-quadrupole mass spectrometry FTIR fourier-transform infrared XRD X-ray diffraction XRF X-ray fluorescence SEM scanning electron microscopy MS mass spectrometry GC–MS gas chromatography – mass spectrometry UV/ViS ultraviolet – visible spectroscopy VM volatile matter (%) FC fixed carbon (%) STC sample temperature controller HHV higher heating value (MJ kg−1) LHV lower heating value (MJ kg−1) Φ carrier gas flow rate (mL min−1) φ* protective gas flow rate (mL min−1) Δm mass of testing sample (mg) Β heating rates (oC min−1) m/z mass to charge ratio SIM selected-ion monitoring TM transmission mode Α conversion degree A the pre-exponential factor (min−1), spectra of products [12–14] can be exploited to extract value-added chemicals by thermal processing, e.g. via pyrolysis. Pyrolysis is thermal degradation of organic material in the absence of (molecular) oxygen. Pyrolysis process is applied on the biomass feedstocks to increase their energy efficiency within physical processes. In general, the carbonization product of biomass is charcoal or simply bio-char. The bio-char is used to produce energy by combustion and gasification processes. In addition, it is used as soil amendment for sustainable soil management. Bio-char is very stable compound that it has carbon negative effect to the atmosphere. It is important for reducing wastes, such as agricultural crops and industrial residues. It should be emphasized that primary reactions from biomass pyrolysis are endothermic and produce gas, tar (oil) and bio-char. In this manner, the extraction of value-added chemicals from the complex bio-oil mixture (tar) is difficult and complex, so the largest number of research is focused on the low-grade gas production (H2, CO, CO2, H2O (water vapor), CH4, lower class of hydrocarbons, CnHm), and production of bio-char, from both, practical and environmental reasons. The aim of this study is the characterization analysis of the products obtained from pyrolysis process of Poplar (Genus Populus alba, Salicaceae family) fluff. The carbonization process was conducted in a fixed bed reactor at static working temperature of 850 °C and with the heating rate of 5 °C min−1. The reactive atmosphere was flowing nitrogen (N2). The characterization of the obtained product (after carbonization) was carried out by applying various analytical techniques such as: XRD (X-ray diffraction analysis), FTIR (Fourier-transform infrared spectroscopy) and SEM (Scanning electron microscopy) for the surface morphology investigation of carbonized material. The simultaneous thermal analysis (STA) which includes TGA (Thermogravimteric

analysis) – DSC (Differential scanning calorimetry) measurements, coupled with Mass Spectrometry (MS) analysis, was used in order to investigate the thermal stabilities of Poplar fluff constituents during pyrolysis process, as well as the temperature effects on studied biomass structural properties. The data obtained from STA were used to investigate the kinetics of pyrolysis reactions especially the devolatilization process, because it is important for the monitoring of commercial pyrolyzers or gasifiers. Above tests were selected in order to check at what the level, the different thermal stabilities of hemicelluloses, cellulose and lignin can provide the opportunities to use the ‘slow’ pyrolysis regimes for a thermal fractionation of Poplar fluff into valuable ended-products. So, in that intention, the GC–MS (Gas Chromatography – Mass Spectrometry) analysis of the raw and carbonized material was performed. The GC–MS analysis was used in order to rapidly analyzing the coarse and carbonized material, and its constituents toward understand their structure and compositions, especially when devolatilization is realized. This research includes the investigation of chemical components (the kinetic study) with a same goal stated from thermal behavior of individual constituents present in Poplar fluff, in order to achieve the best optimization of pyrolysis process. The results obtained in this study can be further used for a more advanced approaches, such as catalytic pyrolysis and catalytic co-pyrolysis that represent emerging methods, for production of high quality pyrolysis bio-oil. On the other hand, the main product of ‘slow’ pyrolysis is the bio-char which has many applications but properties of biochar are decisively affected not only by properties of the parent material, but also by pyrolysis operating conditions, mainly the heating rate, as well as the maximum temperature. For this reason, this study 112

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provides the necessary information about pyrolysis properties of Poplar fluff as new selected fuel, for future considerations about its use on large-scale implementation. The main goals of this research are focused on finding new fuel perspectives through characterization procedures, and biomass upgrading for efficient design of an environmentally sustainable pyrolysis process of a specific lignocellulosic waste.

(the mill sample is passed through a sieve with a hole size of dp ∼ 1 mm (∼1000 μm) (18 mesh sizes sieve)). Prepared sample was divided into seven closely equal portions. First two portions were used for ultimate and proximate analyses while the third portion was used for the simultaneous thermal analysis (STA) measurements. Fourth portion was used for determination of lignocellulosic content and total contents of extractives and mineral matter. The remaining portions were used for further analysis by other analytical experimental techniques in characterization procedure.

2. Experimental 2.1. Material

2.4. The ultimate and proximate analysis

The Poplar fluff (from Populus alba, Salicaceae) is collected from green park of the Capital City, Belgrade, (Block 62) at location: Latitude 44.80299, Longitude 20.37466, N 44o48′10.76036″ E 20o22′28.7657″, Dušana Vukasovića 64, Belgrade, Serbia. The collected raw material was transferred into the plastic bags vacuum sealed and then transported to the laboratory for further testing.

The first portion of prepared sample was used for determination of ultimate analysis (carbon, hydrogen and nitrogen content) according to standard EN ISO 16948 [17]. The results of proximate analysis (total moisture, ash, volatile matter and char content) were obtained from the second portion of the prepared sample, according to standard procedures EN ISO 18134 [18] and EN ISO 17225-1 [19]. Also, the higher heating value (HHV) and lower heating value (LHV) for tested samples were determined using calorimeter laboratory equipment with an oxygen bomb (IKA C200, IKA® Works, Inc., Wilmington, USA), according to the standard EN ISO 18125 [20].

2.2. Statistical analysis 2.2.1. Discriminant analysis We used the discriminant analysis [15] to evaluate the accuracy of the classification procedure. The main objective of using this multivariate statistical technique was to describe differences between the different collection sites (with the same location) and replicates analyzed. Regarding the number of replicates, it should be pointed out that we had technical replicate measurements. In all cases, the data from the three replicate measurements of each type to assess the statistical significance of the difference in the signal responses of the tested sample, we found p > 0.05 (assuming a Gaussian distribution), indicating that they are not significantly different, what was provided unarguable confirmation of sample replicates performance in comparison of the results. The data showed that there are no significant differences in durability among all replicates at the 95% confidence interval (it should be noted that the durability was connected with sample particle size). Autocorrelation is not present (through the implementation of DurbinWatson test) because the data set consists of single-occasion measurements from fluffs that are independent of one another, taken from different sites. Autocorrelation is most commonly encountered when measurements take place through the different time periods. If the autocorrelation was present, it would only be detected if the observations were ordered by time or according to some variable associated with time, which is not present in our case.

2.5. Determination of lignocellulosic content and total quantity of extractives From the fourth portion, the determination of lignocellulosic content (cellulose, hemicelluloses and lignin) as well as the determination of total quantity of extractives was implemented. The content of cellulose was determined by the Kürschner-Hoffer’s method [21]. The lignin content after extraction (toluene-ethanol) was determined by the Klason’s method [22], with an spectrophotometric determination (Specord® Plus UV/ViS Spectrophotometer, Analytik Jena AG, Analytical Instrumentation Konrad-Zuse-Str. 1, 07745 Jena, Germany) of acid-soluble lignin (based on absorption of ultraviolet radiation, with most often used wavelength of 205 nm) according to TAPPI standard method T UM 250 [23]. For determination of extractives soluble in organic solvents, the TAPPI standard method T 264 cm-97 [24] was used. In actual case, a mixture of toluene and ethanol in a volume ratio of 2:1 (C6H5CH3/C2H5OH = 2/1, v/v) was used. The content of extractives soluble in hot water is determined according to TAPPI standard method T 207 cm-99 [25]. The content of hemicelluloses is determined approximately, as a complement to the content of certain components up to 100%. The results are expressed in relation to the absolute dry weight of the studied material, and presented as the mean (arithmetic) value, after four repetitions.

2.3. Raw material preparation and mechanical processing of samples for experimental testing The received Poplar fluff samples were dried at room temperature. After natural drying after 24 h, the Poplar fluff samples were placed inside the oven at 105 °C for 2 h, in order to remove residual moisture that can be sorbed on the outer surfaces. The define amount of moisture was included in the calculation of the total moisture content in the sample. Further, regarding to pre-drying process of the samples, which is necessary to remove the residual moisture during preparation, the samples were placed on a plate and left to reach moisture equilibrium with laboratory atmosphere conditions for 24 h. After that period of time, by the multi-step (two-steps) grinding process was performed. Mechanical treatment was performed using the ultra-centrifugal mill (Retsch ZM-1, Retsch, Gemini BV, Netherlands). In order to avoid the increase in temperature during grinding and in this way damaged the samples, the total milling time of the samples was set at 1 min, with a break interval of 10 s between the two successive grinding cycles. After grinding, the obtained mill sample was treated in accordance with standard EN ISO 14,780 [16] for the solid biomass sample preparation

2.6. Simultaneous thermal analysis (STA) measurements and mass spectrometry (MS) analysis Simultaneous thermal analysis (STA) (TGA-DSC) measurements were performed on the third portion of prepared sample, and all of obtained results were used for studying devolatilization kinetics. NETZSCH STA 445 F5 Jupiter system (Erich NETZSCH GmbH & Co. Holding KG, Germany) was used for STA experimental tests. The inert atmosphere was provided to maintain the pyrolysis process using the high purity argon (Class 5.0) as a carrier gas. At the same time, argon was used as a protective gas in order to keep the high sensitive internal balance (0.1 μg). During all experimental tests, the carrier gas flow rate was set at φ = 30 ml min−1 and the protective gas flow rate was set at φ* = 20 ml min−1. The weight measurements were carried out using the internal balance, which provided the mass results of all testing samples as Δm = 5.0 ± 0.3 mg. The crucibles that were used for all tests are made of alumina, and during each measurement they were

113

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filled approximately up to the half. Crucible’s lid was placed on the top, so the optimum heat transfer could be realized. Each STA sample was tested using three different heating rates (β = 5, 10 and 15 °C min−1). Using the actual heating rates, the STA samples were heated from the room temperature up to 800 °C. During all tests, the DSC (Differential scanning calorimetry) measurements were performed in identical conditions as for TGA signals (same atmosphere, gas flow rate, heating rate, thermal contact to the sample crucible and sensor, etc). All STA has a built-in calibration system-weight for automatic calibration in its weighting system. The STA system is equipped with a noise-free cooling thermostat to guarantee highest long-term stability and low drift behavior. Furthermore, the weighting system is protected from the hot furnace. These chillers can optimally compensate for all fluctuations in the surrounding temperatures. For all measurements, the sample temperature controller (STC) was turned off, so the set temperature (800 °C) is referred to the furnace temperature. Temperatures presented on the diagrams are the sample temperatures so that, due to the construction of the furnace, it’s never reached the set temperature. This fact results in a better temperature curve linearity, than it would be if STC was turned on. The STA 445 F5 Jupiter runs under the versatile Proteus® software and includes all operating tools to obtain a reliable measurement and evaluate the resulting data, or even carry out complicated analyses. Determination of evolved gases from performed STA was carried out continuously using the quadrupole mass spectrometer NETZSCH QMS 403 D Aëolos (QMS) (Erich NETZSCH GmbH & Co. Holding KG, Germany). The STA–QMS coupling was done using transfer line with quartz capillary tube with diameter of 75 μm. The whole transfer line was heated up to 230 °C in order to avoid condensation of evolving volatile compounds. The QMS was operated under vacuum up to 10-7 bar, providing conditions necessary to detect gas components using their ions intensity according to their respective mass to charge ratios (m/z). Evolved gas composition was monitored and analysed through bar-graph cycles, scanning mass units in the range from 1 to 80. Cycles were set to speed of 0.2 s per mass unit and carried out using stair mode. The excitation energy in the QMS was set up at 1200 eV with the resolution of 50 units. From selected range, the focus was on specific ions that correspond to gases evolved in the pyrolysis process. Accordingly, the molecules with atomic mass units (amu) of 2, 16, 18, 28, 30, 44, 58 and 72 which correspond to H2, CH4, H2O, CO, C2H6, C3H8 (CO2), C4H10 and C5H12 respectively, were analysed, and the intensity peak areas obtained for each compound were compared. The screening analysis was performed in the selected-ion monitoring (SIM) mode, in accordance with database of National Institute of Standards and Technology (NIST).

Fig. 1. The raw (the left image feature) and carbonized (the right image feature) Poplar fluff.

2.8. Characterization of raw and carbonized material 2.8.1. X-ray diffraction (XRD) analysis The X-ray diffraction (XRD) analysis was performed on the carbonized material at 850 °C. For characterization of carbonized material, the X–ray Powder Diffraction (XRPD) diffractometer Ultima IV Rigaku (Rigaku Corporation 3-9-12, Matsubara-cho, Akishima-shi, Tokyo 1968666, Japan) was used. The diffractometer was equipped with the Cu Kα1,2 radiation source (a generator voltage of 40.0 kV and a generator current of 40.0 mA), which operates in the range of 5–80 °2θ, with a scanning step size of 0.02 ° and at a scan rate of 2 ° min−1. 2.8.2. Fourier Transform Infrared Spectroscopy (FTIR) analysis Fourier Transform Infrared Spectroscopy (FTIR) is an analytical technique used to identify organic (and in some cases inorganic) materials. The introduction of group frequencies allows the identifications of structural elements of a molecule and makes infrared spectroscopy an important tool for the identification of molecular structure and for quantitative analysis. The FTIR analysis was performed on both, the raw (pulverized) Poplar fluff sample, and bio-char sample which is produced at 850 °C. The FTIR spectra were collected using a PerkinElmer Spectrum two FTIR spectrometer (PerkinElmer, Inc., Waltham, Massachusetts, USA) in transmission mode (TM). The measurements were perfomed on the dry samples where the samples being prepared using the pressed KBr pellets (1:100) technique. A spectrum of KBr in the compartment was used for background correction to remove interfering peaks due to atmospheric water and carbon dioxide. The spectra were recorded in the range from 4000 to 400 cm−1 with the resolution mode of 4 cm−1, in order to obtain the quality spectrum lines.

2.7. Carbonization process The mechanically processed and pulverized material with uniform particle size fraction about 1 mm (1000 µm) was used for conducting the carbonization process. The carbonization was carried out in a stainless-steel fixed bed reactor (Protherm Furnaces, model PTF 16/38/ 250, Turkey). About 20 g of tested material was placed in a carbonization reactor. During carbonization process, the purified nitrogen (N2) at a flow rate of 500 cm3 min−1 was used as purge gas. However, before heating, the system was flushed with dry nitrogen for 30 min to remove all traces of oxygen. The reactor temperature was increased from the room temperature up to desired operating temperature of T = 850 °C. When operating temperature reached the desired value, it was held for 1 h (isothermal conditions). At the end of carbonization, the gaseous flow of N2 in a reactor was maintained during the cooling until the room temperature. The heating rate was constant and its value was β = 5 °C min−1. Fig. 1 shows the appearance of the raw (the left image feature) and carbonized (the right image feature) Poplar fluff samples.

2.8.3. Gas Chromatography – Mass Spectrometry (GC–MS) analysis The following masses of carbonized material 0.02 g and raw material 0.08 g, respectively, was measured in two head-space bottles, and then dissolved in 5 ml of dichloromethane. The closed vials were subjected to extraction on the ultrasonic bath for 30 min, and then from each sample, 1 ml of the total volume was filtered and qualitative analyzed by a gas chromatography technique with a mass detector. Analyses were performed using gas chromatograph (Agilent Technologies Inc, Model 7890B) with mass detector (Agilent Technologies Inc, Model 5977B). The GC column was HP 5-MS (30 mm × 0.25 mm i.d., film thickness 0.25 μm). Helium was used as the carrier gas with a rate of 0.9 ml/min. The sample injector temperature was set at 250 °C, solvent delay is 4 min, and the samples were injected at a volume of 1 µL in splitless mode. The temperature of the 114

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GC/MS transfer line was 280 °C, and the oven temperature was programmed as follows: 40 °C, hold for 2 min, ramped from 40 °C to 180 °C at 8 °C min−1, then 180 °C to 300 °C at 30 °C min−1, hold for 3 min. The mass spectrometry conditions were set as follows: electron ionization at 70 eV, MS source at 230 °C and MS quad 150 °C, stored mass range: 50–550 m/z. The instrument library for detection of identified compounds was used and there was implemented in results.

where the i-th pseudo-component of biomass decomposes, αN represents the total conversion degree that composes all αi values for N reactions present, while φi is the contribution of the partial reaction to the overall process (the definition of this parameter is dependent on the concentration or mass fraction of the i-th pseudo-component in considered sample). The kinetic parameters φi, Ai, Ea,i and ni for each pseudo-component in Poplar fluff devolatilization can be obtained by minimizing the objective function (OBF) described by the following equation:

2.8.4. Scanning Electron Microscopy (SEM) analysis Scanning electron microscopy (SEM) was used to evaluate the morphology of initial (raw) material and obtained bio-char at 850 °C. The morphology was observed using the scanning electron microscope JEOL JSM-5800 (JEOL, Ltd., Akishima, Tokyo, Japan). The tested sample was placed in a carbon strip (which is sticky on both sides - on one of sticky side with tweezers, the studied sample was placed), which is then entered (with side where the sample was not submitted) to carrier of device, and then carries out corresponding measurement. The instrument provides the quality images with a high resolution.

Nk

)n ,

Nm

QOF = 100 × m=1

i Ai ·exp

Ea, i ·(1 RT

(1)

n i ) i,

(2)

and d N dt

=

N

=

N d i , i = 1 dt N , i=1 i

, calc, k , m

(4)

( )

d N dt exp, m

( )

d N dt calc, m

Nm

max

d dt exp, m

2

, (5)

where m is the number of the data points, max |dα/dt|exp,m is the maximum experimental value (the highest value of decomposition rate registered during the experiment). Considering the mathematical formulation of the multi-component kinetic modeling, in order to solve all presented equations, defining the first prediction of all kinetic parameters φi, Ai, Ea,i and ni for each pseudo-component was necessary. Values for the first prediction were obtained according to results of the isoconversional kinetic method (model-free method) and were used for the start of iteration process. In this work, we used the differential Friedman’s [FR] model-free method [30] for starting predictions. The basic equation of FR method has a form ln(dα/dt)α,j = ln[Aα·(1 − α)n] − Ea/RTα,j and can be applied in combination with the assumption of a first order (n = 1) or n-th order (n ≠ 1) reaction, with the reaction mechanism function in an analytical form of (1 − α) and (1 − α)n, respectively. From this equation, the simple estimation of lnA values can be performed. In each iteration step, lsqcurvefit function for solving non-linear curve-fitting was set to use trust-region-reflectiv algorithm, with function and step tolerance equals to the 10-12 and the maximum function evaluations equals to the 103. The criteria for the final solution of the calculations according to the results of the kinetic parameters for each pseudo-component was the minimum value of QOF given by Eq. (5). For the model selection, the Akaike information criteria (AIC) were used [31]. The AIC assesses model fit and model parsimony (AIC score allows the judging of suitability of model selected). When model fits are ranked according to their AIC values, the model with the lowest AIC value being considered the ‘best’. In this procedure, we only present the model with lowest AIC value.

where the Eq. (1) represents the basic rate-law expression for a singlestep reaction model, where process proceeds with n-th order reaction mechanism. Thermal decomposition of any biomass specie can generally be attributed to its major components hemicelluloses, cellulose and lignin decomposition. As a result, biomass decomposition is the sum total of all multiple pseudo-components decomposing during thermal events. Consequently, the kinetics of multi-component devolatilization of Poplar fluff can be modelled by assuming pseudo-components decompose as independent and parallel n-th order reactions, such as:

d i = dt

d N dt

exp, k , m

where Nk and Nm are the number of heating rates and the numbers of TGA measured, respectively. The procedure was performed using the multi-dimensional nonlinear regression function lsqcurvefit in MATHLAB R2016b. This function is commonly used for solving non-linear curve-fitting (data-fitting) problems in the least-squares sense. Conversely, minimizing the Eq. (4) requires mathematical evaluation of the Eq. (3) for all experimental temperature ranges and iteration process which was performed by while loop in MATHLAB R2016b. Since the simulation considers a good number of pseudo-components (the analysis was performed for 3–5 pseudo-components), the model quality of the fit is determined by Eq. (5) for each overall simulation process and their heating rates [29]:

In order to observe and understand the thermal decomposition of biomass, the various global and semi-global models that utilize a limited, yet sufficient number of successive and parallel reactions in order to describe this process have been developed. These include models that deal only with chemical transformation from one phase to another (i.e. solid to gas, and solid to liquid reactions) [26,27], while others consider biomass as a mixture of its main constituents, such as, cellulose, hemicelluloses and lignin, and employ reactions for the chemical conversion of these [28]. The advanced method which includes application of the optimization algorithm that allows the complete kinetic information related to the kinetic modeling based on the multi-component biomass analysis was applied in this work. Devolatilization kinetic model applies the Arrhenius rate of decomposition for a solid state equation. Rate-law equation relates conversion degree (α), the pre-exponential factor (A), the apparent activation energy (Ea), temperature (T), the gas constant (R) and the time (t), can be expressed as:

Ea ·(1 RT

d N dt

k=1 m=1

3. Multi-component kinetic analysis

d = A·exp dt

Nm

OBF =

(3)

where dαi/dt is the rate of i-th pseudo-component decomposition (considering that dαN/dt ≡ βj·dαN/dT, reflecting the presents of j-th heating rate), Ai is pre-exponential factor of i-th pseudo-component, while Ea,i and ni represent apparent activation energy and reaction order related to the decomposition of i-th pseudo-component of biomass, respectively; αi is the conversion degree values within the range

4. Results and discussion 4.1. Proximate, ultimate and component analyses Through the detailed literature review, according to the proximate and ultimate analysis, the Poplar fluff is most closely related to the 115

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Table 1 Comparative presentation of proximate/ultimate analysis between Poplar fluff and Cotton seed samples. Sample

Source

HHV (MJ kg−1)

Poplar fluff Cotton seed

This work [32,33]

15.86 23.08

e

LHV (MJ kg−1)

Moisture (%)

Ash (%)

VM a (%)

FC a (%)

C (%)

14.20b 22.75

9.41 10.05

7.67 6.00

66.67 67.95

16.25 16.00

38.52 53.10

c

H (%) 6.35 3.55

c

O (%)

c,d

33.29 37.34

N (%) 4.75 4.47

c

S (%) 0.02 1.54

c

H/C 1.98 0.80

c

O/C

c

0.65 0.53

a

VM and FC represent the volatile matter and fixed carbon, respectively. Calculated according to [20]. c On a dry basis. d By the difference. e Obtained HHV value is in the range which corresponds to HHV’s of lignite/brown coal (fossil) fuel, in compliance with IEA (International Energy Agency) definition (< 17.40 MJ kg−1) (Literature source – Heat values of various fuels: http://www.world-nuclear.org/information-library/facts-and-figures/heat-values-ofvarious-fuels.aspx. b

Cotton seed. Table 1 shows comparative review according to a proximate and ultimate analysis between the raw samples of Poplar fluff and Cotton seed. From presented results (Table 1), it can be observed that Poplar fluff and Cotton seed have about the same moisture content, as well as the ash content. The greater presence of moisture may influence significantly on the thermal decomposition degrees of studied biomass during pyrolysis. Thermal conversion technologies can use feedstocks with high moisture content but the overall energy balance for the conversion process is adversely impacted. On the other hand, the ash content is an important parameter which directly may affects on the heating value. So, the high ash content of the plant part biomass makes it less desirable as fuel, whereas the high extractive content adds to its desirability (Table 2). In addition, Poplar fluff belongs to the category of the lignocellulosic material with low lignin content (Table 2), but with high cellulose (fibrillose) content. From pyrolysis properties, the biomass with higher cellulose content may cause the faster pyrolysis rate. For Poplar fluff, the lower lignin content affects the appearance of the lower value of HHV, along with increased ash content, in comparison with wood species (such as Pine, Fir or Beech wood), as well as with wetland herbs [33]. Compared with the HHV value for Cotton seed, the higher heating value of Poplar fluff (Table 1) is still in the range of HHV values obtained for agricultural residues and wastes [33]. In general, taking the results presented in Table 1, the Cotton seed has higher energy values compared to Poplar fluff or other plant organs (usually for seeds, HHV = 22.75–23.08 MJ kg−1, and flowers, HHV = 17.72–18.56 MJ kg−1) might be due to the higher lipid content, which reflects optimal environmental condition for the plants. The LHV value for Poplar fluff is also in the range of LHV values characteristic for agricultural residues and wastes (Table 1) [33]. The LHV of the fuel increases with an increasing of the sulfur content due to the cause SOx gases absorbed by water, but in actual case does not occur due to the negligible content of sulfur (Table 1). However, in the practical terms, the latent heat contained in the water vapor cannot be used effectively, and therefore, the LHV is the appropriate value to use for the energy available for the subsequent use.

The higher ash content is present for both, Poplar fluff and Cotton seed, and increased ash content can affects on reducing of the volatile matter (VM) (Table 1). However, the effect of VM content on HHV is much more complicated and inconclusive. High VM does not guarantee a high calorific value since some of ingredients in VM are formed from non-combustible gases, such as carbon dioxide and water. Poplar fluff (as well as Cotton seed) is not characterized by an extremely high FC (fixed carbon) content (Table 1), where lower fixed carbon is more appropriate for the case of volatile intensification to syngas, and opposite, the higher fixed carbon is good indication to maximize the biochar yield (and carbon sequestration). The Poplar fluff has a lower O/C ratio (O/C = 0.65), which means that represents the material that have more energy density which was almost the same as for Cotton seed (Table 1). In general, the higher proportion of oxygen and hydrogen, compared with carbon, reduces the energy value of the fuel, due to the lower energy contained in carbonoxygen and carbon-hydrogen bonds, than in carbon-carbon bonds. Since that H/C ratio for Poplar fluff is higher (the higher energy content) than for Cotton seed, this fact is associated with increased content of fibrous structure, i.e. cellulose (Table 2). Besides cellulose, the higher H/C ratio can be a consequence of the elevated presence of more residues of the organic matters such as carbohydrates and the fatty acids. In this manner, elevated H/C ratio also means the higher portion of hydrogen molecules in tested fuel. Having this in mind, the hydrogen has the highest burning velocity between all fuels weather gases or liquids. Increasing hydrogen portion in the fuel combination means that the fuel burning velocity will be better and cleaner as burning hydrogen gives only water. It can be seen from Table 1 that the H/C ratio for Poplar fluff is almost twice higher than the H/C ratio identified for Cotton seed. Further, the lignin represents an important target for the pyrolysis process, since the major lignin-derived products usually have a lower O/C ratio, manifesting the higher energy value, and a more stable than the sugar-derived products. In addition, the increase in the relative carbon content and decrease of other elements, such as O, H, N and S, can result in a decrease in the volatile matter fraction. Generally, the H/C atomic ratio generally decreased with heating temperature, which was consistent with the reported increasing aromaticity because H/C was an index of aromaticity [34]. So, the high pyrolysis temperature probably would produce a decline in H/C ratio and it can be favored high temperature derived bio-chars (> 400 °C), and then the precursors mainly experienced the aromatization processes and generated tiny aromatic cluster graphene-like structures [35]. It can be assumed that heat treatment of Poplar fluff may causes decreasing of O/C ratio with an increasing of temperature. Namely, this assumption may improve the decomposition of hemicelluloses and eventually regeneration of lignin, since on the ratio of these two pseudo-components in Poplar fluff (Table 2). Lower atomic ratio O/C and higher atomic H/C ratio in the raw material may suggest that aliphatic carbon containing compounds may decrease and aromatic

Table 2 Lignocellulosic content and content of extractives of raw Poplar fluff sample. Poplar fluff sample Cellulose content (%) Hemicelluloses content b (%) Lignin content (total) (%) Content of soluble extractives in hot water (%) Content of soluble extractives in an organic solvent (toluene/ethanol = 2:1) (%) a b

41.85 ± 0.64 12.30 ± 0.30 24.04 ± 0.83 16.68 ± 0.33 5.13 ± 0.12

a

Standard deviation. By the difference. 116

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formation of N2. Other part of nitrogen remains in bio-char [36]. So, the two main disadvantages of Poplar fluff feedstock from energy/fuel aspects represent the significant presents of ash and nitrogen (Table 1), which may raise the NOx. In this manner, the discussed remaining inorganic part (ash) is important from storage and transportation aspects. Furthermore, the special attention must be paid to alkali, which plays a major role in ash deposition, bed agglomeration, hot corrosion, and particle emissions. Si, K, Mg are important, too, for the properties of the formed ash. The reaction of alkali metal with, for example, the silica present in the ash, may produces a sticky, mobile liquid phase that can lead to blockages of airways in the furnace and boiler plant. Heavy metals must be taken into account for an approach to the environmental issues raised by the process. Therefore, an additional identification analysis was carried out in connection with these issues, by the application of X-ray fluorescence (XRF) spectrometry (was not included in the main text) performed directly on the raw Poplar fluff sample. The results of this analysis was presented in a form of table (Table S1) and presented in Supplementary material. It can be observed that the elements positively detected by the XRF are Mg, Si, P, S, V, Cr, Fe, Zn and Cu. The raw material has the highest concentration of magnesium, then silicon and then iron and chromium (Table S1). In this case, the high Mg ash content can play an important role in catalytic process in high temperature obtained bio-char and especially with reactions with CO2. The Poplar fluff contains relatively high concentrations of Si and Fe, which can help in preventing the formation of chlorides, which can be formed from relatively high concentrations of alkali/earth alkaline metals in the wood-based materials. High percentage of ash can have further influence on the product yields and on the characteristics of biooil and bio-char obtained in the pyrolysis. The high ash content may provides a larger, potentially catalytically active surface on which secondary tar polymerization and bio-char forming reactions can take place.

Fig. 2. Correlation between carbon content and HHV of the raw materials, Poplar fluff (this study), Poplar wood, as well as agricultural and crop residues (as reference).

compounds may increase during gasification process. Fig. 2 shows the correlation between carbon (C) content (%) and HHV (MJ kg−1) of the raw materials, considering Poplar fluff together with Poplar wood species, as well as agricultural and crop residues. A nearly linear correlation between carbon content and HHV is exhibited in Fig. 2. The samples with higher carbon density have higher HHV’s, including Cotton seed, White Poplar, Bagasse and Cotton sticks (Table 1 and Fig. 2). Based on the correlation shown, the Poplar fluff belongs to the group of biomass samples, such as Rice husk and Rice straw within herbaceous biomass fuels (in Fig. 2 this is indicated by the region limited by two curved arrows). Considering C and HHV values, only Poplar fluff and White Poplar almost lie on the same correlation line in two different carbon content zones (Fig. 2) indicating their common biological origins. The HHV increases with C contents, consistent with commonsense that the higher C contents mean a higher energy content of studied biomass. Poplar fluff has a disadvantage in terms that possessed a higher content of ash in the same manner with a raised ash content identified in the case of Cotton seed (see Table 1). Therefore, a significant contribution to the ash of Poplar fluff as possible fuel injector for the heat boilers is a notable lack of studied sample. Particularly, when this type of biomass is used in a boiler, it must be originally designated for this tested fuel, so that availability may be seriously hampered. It is therefore essential that proper fundamental understanding exists of the ash chemistry in a boiler, and that the particularities of a fuel are taken into account seriously, when designing or even modifying a new boiler. In general, the tested biomass fuel should be within a specified range of properties to ensure good combustion and to minimize slagging and fouling and corrosion in the plant. On the other hand, related to mentioned issue, the Polar fluff contains a higher percentage of nitrogen (4.75%, Table 1). Therefore, the special attention can be paid to NOx emissions from combustion of nitrogen-containing biomass fuels. The NOx emission from combustion of a nitrogen-containing fuel comes from two sources: thermal NOx and fuel NOx. The former is formed from the nitrogen in the combustion air and its formation is more or less dependent on the temperature and pressure in the combustor. The latter comes from oxidation of nitrogen in the fuel and is not particularly temperature sensitive. All the nitrogen oxides also enhance the greenhouse effect. During gasification, the fuel-nitrogen mainly forms NH3. During the combustion of the gases, ammonia and cyanides undergo oxidation to NOx. The pyrolysis process is the initial step in both gasification and combustion. During pyrolysis part of the nitrogen is converted to ammonia (the main product), hydrogen cyanide (HCN), and nitric oxide (NO). The conversion of nitrogen may also leads to

4.2. STA-MS analysis of Poplar fluff pyrolysis process Fig. 3 a-c shows TGA-DTG-DSC curves of slow pyrolysis process of Poplar fluff feedstock recorded at various heating rates (β = 5, 10, 15 °C min−1) in an argon atmosphere. It can be observed from TGA curves (Fig. 3 a) that three reaction stages of pyrolysis process exists. The first one represents the pre-stage, and this corresponds to extraction of moisture and adsorbed water in the Poplar fluff sample. The pre-stage occurs in the temperature region of ΔT1 = 25–175 °C. The water evaporation region was indicated on DTG curves by “1” (Fig. 3 b). The removal of moisture and hydrolysis of some extractives can be expected up to approximately 175 °C, which is followed by minor mass loss (∼9.51%; Fig. 3 a). After 175 °C, the studied sample becomes significantly unstable, and about 62.64% of the mass loss (Fig. 3 a) occurs in the temperature range of ΔT2 = 175–500 °C, which represents the active (main) pyrolysis stage (marked by “2” in Fig. 3 b). In indicated temperature range, the most of gaseous products are liberated, where usually reach its maximum between 250 °C and 300 °C, and virtually ceases about 350 °C. The tar forms between about 280 °C and 400 °C up to 450 °C. The last stage (marked by “3” in Fig. 3 b) takes place above 500 °C and corresponds to the high temperature charring of the residue followed by the much smaller mass loss (∼3.56%; Fig. 3 a). Still, there remained a little residue at 790 °C, i.e. about 24% of the original mass. However, charcoal which contains practically all of the original ash, in some cases (depending on the presence of the lignocellulosic content of biomass), is not completely carbonized even up to the very high temperatures (e.g. 1500 °C). As can be seen from Fig. 3 b, the DTG curves at all heating rates exhibit a single peak at about 335–350 °C. It can be observed that maximum peak height does not vary with the increase in the heating rate, but the peaks move to the side of the higher temperatures laterally. Also, it can be noticed that there is no additional 117

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opposite to the Poplar fluff pyrolysis (Fig. 3 b). The heat transfer phenomena for Poplar wood pyrolysis is very pronounced, which is not the case for Poplar fluff pyrolysis. In considered work [38], the DTG features are typical for wood pyrolysis which can be distinguished the decomposition regions for cellulose and hemicelluloses contents. Namely, this can not be clearly seen on the DTG curves of Poplar fluff pyrolysis. However, the rate of breakdown and product states is determined by the order of thermo-chemical stability of individual biomass constituents where hemicelluloses decomposes much faster in the temperature range 175/200–300 °C, as the least stable polymer, to the more stable polymer, cellulose, which decomposes from 300 up to 400 °C, while lignin usually exhibits intermediate thermal decomposition behavior (in the temperature range of 250–500 °C; likewise the DTG curves become flatter above 550 °C, which reveals a slower decomposition that also can be attributed to lignin pseudo-component). The single set of DTG peaks at all heating rates falls within the largest mass losses identified on TGA curves (Fig. 3 a), where this behavior is related mainly to cellulose decomposition process. Namely, such phenomena are not unusual considering that the Poplar fluff contains high percentage level of cellulose (Table 2), while the remaining two constituents by content are relatively low. Consequently, the Poplar fluff is by the composition most similar to the cotton and cotton-based materials [38]. For example, the DTG curve for slow pyrolysis of cotton residue [39] shows the single significant decomposition step with a peak temperature about 328 °C, which is very similar to the behavior of DTG curves for slow pyrolysis of Poplar fluff (Fig. 3 b). So, the thermal decomposition of cellulose during biomass pyrolysis is very important because produces solid residues, liquid materials and volatile gases. The pyrolysis of cellulose is a very complex chemical process and is commonly believed to involve two different mechanisms [40]. One of them is a process of dewatering and charring of cellulose, producing water, carbon dioxide and solid residues. While according to the second mechanism, the cellulose produces nonvolatile liquid L-glucose by the depolymerization, and L-glucose cleavage continues, producing low molecular weight products. However, the competition of these two reactions exists throughout the thermal decomposition of cellulose. In addition, the DSC signals may provide changes of temperatures of the material in the pyrolysis process and the changes of heat releasing during the same process. From DSC curves of Poplar fluff pyrolysis process at considered heating rates (Fig. 3 c), we can see occurrence of small endothermic effect in the low temperature region up to 100 °C, which can be attributed to water evaporation (“I”; Fig. 3 c). The main pyrolysis stage occurs in the temperature range of 300–400 °C and most of pyrolysis products are produced in this stage. The mentioned stage is characterized by the endothermic effects (“II” Fig. 3 c) through decomposition reactions, where all kinds of combustible gases can be released together with anhydro-sugars, as well as C1–C4 oxygenates (such as methanol, formaldehyde, formic acid, acetone, lactones, etc). The char pyrolysis occurs at the temperature above 450 °C, which is followed by endothermic effect about 505–520 °C (“III” Fig. 3 c), where the process continues with the decarboxylization, releasing more water and carbon dioxide and producing double bond, carboxyl and carbonyl products. In a high temperature region (above 600 °C), a less pronounced exothermic effect can be observed (“IV” Fig. 3 c), and this can be attributed to the charred residue formation. In this stage, the carbon content in the decomposed products becomes higher and higher. Based on these results, it can be expected that decomposition of cellulose during Poplar fluff pyrolysis take place from amorphous regions to crystalline regions in cellulose molecules. Fig. 4 a-c shows simultaneous thermogravimetry (TG) – mass spectrometry (MS) pyrolysis profiles, through examination of the evolved gaseous products during the process, at the heating rate of β = 10 °C min−1. Generally, from presented results in Fig. 4 a-c, it can be observed that principal of the maximum rates of evolution of gaseous products occurs in the temperature range of 300–500 °C.

Fig. 3. TGA-DTG-DSC curves of slow pyrolysis process of Poplar fluff at different heating rates.

peak or occurrence of the “shoulder” in the active pyrolysis zone, attributed to the decomposition of hemicelluloses, which is typical for DTG curves attached to pyrolysis of agricultural residues and wood/ woody biomass feedstock [37]. In addition, the declining of DTG curves with an increasing of heating rate was not detected (Fig. 3 b), which means that the thermal-lag effects are reduced to a minimum. These results show that all three chosen heating rates used for pyrolysis monitoring are fully representative in order to obtain optimal heating conditions without moderating. The thermo-analytical profiles (TGA and DTG curves) for pyrolysis process of Poplar fluff are different from the ones related to Poplar wood pyrolysis [38] at the same heating rates. Namely, in the case of Poplar wood pyrolysis, the heating rate significantly affects on TGA curve positions, maximum decomposition rate, as well as location of peak maximums [38], which was not identified in the case of Poplar fluff pyrolysis. For Poplar wood, the DTG curves are significantly shifted to higher temperatures [38], which is 118

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a

products which may originate by the lignin are derived from guaiacyl (G) or syringyl (S) subunits, because the Poplar species were richer with lignin that contains these subunits [41]. The CO is the product of ether groups present in lignin and/or ether compounds produced in the secondary cracking of volatiles. It should be noted that MS peak for the CO (at 350 °C) coincides with DTG peak (Fig. 3 b) at the same value of the peak temperature. In addition, the beginning of the formation of the “shoulder” at about 385 °C corresponds to the DTG maximum peak attributed to the lignin maximal rate of decomposition (∼387 °C), characteristic for woody sample [42]. This fact further confirms the above views. Similar situations with the evolution of the CO are with the evolution profile of CO2, but its intensity of evolution is considerably lower than for CO. The CO was produced from the CeOeC functional groups which were abundant in the structure of cellulose (which is also preliminarily highlighted above). The peak for CO2 evolution coincides with a pyrolytic water peak evolution (Fig. 4 c), also about 350 °C. So, the water can be formed by cracking, the reaction of oxygen functional groups at higher temperatures, or by the dehydration of the hydroxy group in cellulose and hemicelluloses. The CO2 formation can be a consequence of the thermal decomposition of eCOOH and CeO groups and maybe the water gas shift reaction. The residual release of CO2 starting about 400 °C is probably linked with the polymerization reaction of coking in the solid phase. Namely, as an important fact in this study, the content of extractives can significantly affect on the increased release of water, CO2, and CO (Fig. 4 c), but on the decreased release of alkanes (Fig. 4 a-b). The decomposition of cellulose produces saccharides and CO at the main temperature zone of 315–400 °C with the formation of active cellulose at the range of approximately 250–270 °C. Hemicelluloses decompose at low temperatures, and obtain maximum volatiles release at about approximately 325 °C. In addition, the pyrolysis products like methane are more complex than cellulose due to the secondary cracking of volatiles, and may also originates from lignin decomposition (see one broader peak evolution for CH4 at about 500 °C, Fig. 4 a). Taking these facts into considerations, the extractives should be addressed if their content in biomass sample is higher than 10% [43]. The Poplar fluff contains a high percentage of extractives which was greater than 20% (Table 2). The existence of extractives may enhance the decomposition of lignin to form phenolic homologues, without obvious thermal difference between the original biomass and the extracted residues. Furthermore, the formation tendency of CO and alkanes is generally similar, and “extractive extracted” biomass gives more inorganic compounds, e.g. water, CO, and CO2 (Fig. 4) and usually similar yields of aldehydes and alkanes (Fig. 4 a-b). In addition, the existence of extractives can catalyze the formation of acidic compounds, and its absence may enhance the water formation. Considering the current facts, it can be assumed that the levoglucosan formed during cellulose decomposition (through chain-end depolymerization reaction) may appear about 350/400 °C, where water (H2O) and CO2 (Fig. 4 c) can be formed by secondary cracking of levoglucosan. The eventually presence of metal salts, may decreased the release of small-molecular gaseous products by inhibiting levoglucosan to crack due to the blockage of free junction of cellulose chains and reversely enhanced the formation, for example, the acetic acid. The formation of CO2 is mainly related to the decarboxylation of acetic acid, and CO is mainly formed from the decarbonylation of aldehydes, such as glycolaldehyde, formaldehyde, and acetaldehyde. The H2 (Fig. 4 a) originates primarily from the cracking and deformation of C]C and CeH bonds, while the CH4 (Fig. 4 a) is produced from decomposition of OeCH3 groups [44]. In addition, the gaseous C3 products and other hydrocarbons can also be formed by the secondary decomposition of volatiles [45]. Consequently, all these results are in agreement with previous findings, that the most pyrolytic products should be stimulated by the decomposition of the cellulose fraction of the studied biomass.

b

3

c

Fig. 4. TGA – MS pyrolysis profiles of Poplar fluff sample: a – H2, CH4 and C2H6 evolution profiles, b – C4H10 and C5H12 evolution profiles, and c – H2O, CO2 (C3H8) and CO evolution profiles.

From the mass spectrometry analysis, it can be observed that CH4, H2, CO, CO2 and C4-C5 hydrocarbons are the major gas products. Namely, the both, CO and CO2 (oxygen-containing gases) were mainly produced from thermal decomposition of cellulose and hemicelluloses, where at higher temperatures the CO releasing arises from the cracking of phenolic, carboxyl and oxygenic heterocyclic compounds probably situated in the tar. The evolution of CO increases from about 200 °C and reaches its maximum at about 350 °C, and then progressively deceases until the end of monitored process. It can be observed that CO evolution profile (Fig. 4 c) shows a certain deviation from the DTG profile (Fig. 3 b) at the same rate of heating. From actual MS profile, the CO evolution feature exhibits one peak and one additional “shoulder” at about 450 °C (Fig. 4 c). It means that the CO evolution does not only originate from the cellulose but also from lignin decompositions, which starts to significantly degrade at 300 °C. However, the major volatile gaseous

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4.3. Multi-component kinetic modeling Kinetic models based on the “summative” principle are generalizable to a wider range of biomass in which the overall decomposition of biomass is the weighted sum of the individual decomposition of its three main components, i.e. cellulose, hemicellulose and lignin. Therefore, a combination of the individual mechanisms of its main components is used, each mechanism with a specific set of the kinetic parameters. The multi-component mechanism is suitable for analyzing the biomass pyrolysis, as long as the biomass is properly characterized, since the amount of its components is the starting point for suitable process analysis. The rate of pyrolysis of one biomass type can be represented by the sum of the corresponding rates of the main biomass components (pseudo-components such as cellulose, hemicelluloses and lignin). The kinetics of each of these pseudo-components can be simulated by the kinetic scheme which was capable for predicting the pyrolysis rate, and the final mass loss for a wide range of pyrolysis parameters, also including and the various heating conditions. Based on the multi-component kinetic analysis, the computational procedure provides a four-component reaction model (the best selected model within optimization algorithm is the four pseudo-component biomass reactivity model) in describing the Poplar fluff pyrolysis. The approach results in accurate determination of kinetic parameters for each decomposition reaction step, with a generated overall rate curve. The algorithm assumes the presence of n-th order reaction mechanisms. This means that the model allows description of the global decomposition by different number and type of pseudo-components and n-th order kinetics for the partial reactions. The deconvolution curves of the pseudo-components (pseudo-component was marked by “PSC”) from the pyrolysis kinetic modeling of Poplar fluff sample at various heating rates, are shown Fig. 5 a-c. Fig. 5 a-c also shows the generated curves from the applied model (the continuous full red-line). The four pseudo-components are the following: PSC-1: cellulose, PSC-2: hemicelluloses, PSC-3: extractives, and PSC-4: lignin, respectively. The fit, as well as model results are presented through QOF and AIC values (Table 4) show a very good agreement between experimental and model calculated data for all observed heating rates of the conducted experiments. Regarding the investigated material in this work, the decomposition domains, the shape and width of the pseudo-components derived from the model are similar to devolatilization partial rate curves of hemicelluloses, cellulose, extractives and lignin in lignocellulosic materials that possess these constituents. It was found that total area beneath each deconvoluted curve corresponds to the following order of pseudo-components: PSC-4 < PSC2 < PSC-3 < PSC-1, where the total area is the largest for cellulose decomposition, while the total area is the smallest for the lignin decomposition. The temperatures range where the maximum (peak) rate of decomposition of the individual pseudo-components occurs at all the heating rates were identified and follow: ΔTpPSC-1 = 330–350 °C (cel– 245 °C (hemicelluloses), ΔTpPSClulose), ΔTpPSC-2 = 230 3 = 420–430 °C (extractives) and ΔTpPSC-4 = 610–630 °C (lignin). Namely, the hemicelluloses (ΔTpPSC-2) decompose early and this is agreement with literature reports (in a temperature range of 200–310 °C) [46] depending on the biomass feedstock. The cellulose is decomposed in a temperature range of 290–400 °C with a maximum decomposition rate observed at 349 – 355 °C [46], where ΔTpPSC-1 is in good agreement with a given data, which also depends on its content in the studied biomass. The lignin decomposed slowly in a wider temperature range (from 200 °C to even 900 °C) [46], and ΔTpPSC-4 values are within indicated decomposition temperature range related to lignin. The variation of these pseudo-components across the various pyrolytic temperature ranges is caused by their different responses to the pyrolytic treatments. Extractives are decomposed in a higher temperature reaction zone, where obviously extractives significantly affect on the overall decomposition kinetics.

Fig. 5. Deconvolution curves regarding the individual pseudo-components devolatilization at the heating rates of 5, 10 and 15 °C min−1 for pyrolysis process of Poplar fluff, using independent parallel reactions model, by optimization procedure.

Namely, based on the total area contributions and elevated extractives content in Poplar fluff, devolatilization of extractives probably occurs with overlapping in temperature zone of hemicelluloses decomposition. Intensity of maximum decomposition rates of these two pseudo-components is almost equal at all heating rates (Fig. 5 a-c). The mineral matter, especially alkalis (primarily magnesium) which in greater quantity can substantially influence on the decomposition mechanism and the products distribution, inhibiting the volatiles formation, and increasing the bio-char yield [47]. The presence of magnesium additives can promote the trends that non-condensable gases such as H2, CH4, CO and CO2 evolved much earlier. The presence of magnesium compounds in starting material (Table S1, Supplementary material) may shift some of the cellulose decomposition reactions to lower temperatures, which may indicates on the certain catalytic effects. In this case, the cellulose decomposition fraction can significantly contribute to the increase of the bio-char formation. On the other hand, 120

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the higher water content can significantly favored the formation of biochar, where the water molecules play the role of a catalyst. Table 3 lists the values of devolatilization kinetic parameters together with values of contribution of the partial reactions, as well as the quality of the fit parameters. It can be observed that intensity of volatile species expressed through contribution of partial reactions (φ) varies with heating rate, but their absolute values do not deviate much from each other. It is possible that eventually catalytic effects caused by inorganic species such as ash, may cause that the separation of the decomposition of hemicelluloses and cellulose be much less pronounced as shown in DTG curves (Fig. 3 b), and this is reflects on the generated pyrolysis rate curves (Fig. 5), which would be different from the behavior of conventional biomass samples. Given that the studied material consist the higher cellulosic fraction, it indicates that the catalytic effects may be present. However, in the kinetic evaluation of the Poplar fluff fractions, the four observed decomposition reactions separate from each other in different ways were caused by the differences in their chemical structures. It can be noted that total peak area for cellulose decomposition rapidly increases with an increasing of heating rate, in comparison with other pseudo-components (the total areas increases for them too). From presented results (Table 3) considering all heating rates, the kinetics of decomposition of Poplar fluff pseudo-components is departed from specific and simple first-order reaction model, and obeys to more complicated reaction mechanisms with n ≠ 1 (this is not unusual behavior given the complex chemical structure of biomass, and the large variety of chemical linkages). For different pseudo-components decomposition, the n value varies with the heating rate, but all values are smaller than 2, which prove that the reproductivity of the rate

Table 3 The kinetic parameters for independent parallel reactions model derived for Poplar fluff slow pyrolysis process. 5 °C min−1 Parameters

PSC-1

PSC-2

PSC-3

PSC-4

φ A (min−1) Ea (kJmol−1) n QOF (%) AIC

0.2835 6.721 × 105 79.4 1.70 1.3557 −2437.04

0.2452 2.180 × 103 65.4 1.31

0.2228 8.452 × 103 107.6 1.06

0.2485 1.758 × 104 56.7 1.18

Parameters

PSC-1

PSC-2

PSC-3

PSC-4

φ A (min−1) Ea (kJmol−1) n QOF (%) AIC

0.2521 5.123 × 103 67.9 1.28 1.2890 −2172.55

0.2590 1.407 × 102 73.6 1.07

0.2340 3.149 × 104 57.1 1.17

0.2550 7.916 × 105 77.9 1.79

Parameters

PSC-1

PSC-2

PSC-3

PSC-4

φ A (min−1) Ea (kJmol−1) n QOF (%) AIC

0.2324 1.038 × 102 68.4 1.07 1.1640 −2069.43

0.2597 9.322 × 105 78.2 1.72

0.2451 1.841 × 103 58.7 1.34

0.2629 6.334 × 103 49.2 1.18

10 °C min−1

15 °C min−1

Table 4 The main organic components of bio-oils with GC–MS analysis. No.

RT (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

5.975 6.175 6.638 7.737 8.092 8,761 8.944 9.620 9.688 9.997 10.26 10.86 12.13 12.53 12.66 12.86 12.75 14.42 14.52 14.82 14.86 15.94 16.20 16.33 16.38 17.59 17.67 18.15 19.24 19.79 20.44 23.07

a

Retention time. Obtained at 280 °C.

b

a

Compound

Class of the organic compounds

2 Hexanone, 5-methyl p-Xylene 2-Heptanone 1-Hexene, 4,5-dimetyl Benzaldehyde Benzene, 1,2,4-trimethyl Octanal Indane 2-Pyrrolidinone, 1-methyl Nonane Acetophenone Undecane Acetic acid, phenylmethyl ester Naphthalene Cis-4-Decenal Decanal Dodecane 2-Undecanone Naphthalene, 1-methyl 1H-Indene, 1-ethylidene Phthalic anhydride Biphenyl Tetradecane Diphenyl ether Naphtalene, 1,4-dimethyl Naphthalene, 1-bromo Acenaphtene Dibenzofuran Diethyl Phtalate Benzophenone 2-Dodecanone Hexadecanamide

Ketones Benzene and substituted derivatives Ketones Alkenes Benzoyl derivatives Benzene and substituted derivatives Medium-chain aldehydes Indanes Pyrrolidines Alkanes Alkyl-phenylketones Acyclic alkanes Benzyloxycarbonyls Polycyclic aromatic hydrocarbon Medium-chain aldehydes Medium-chain aldehydes Alkanes Ketones Polycyclic aromatic hydrocarbon Heterocyclic organic compounds Anhydride of phthalic acid Biphenyls and derivatives Alkanes Diphenyl ethers Polycyclic aromatic hydrocarbon Polycyclic aromatic hydrocarbon Polycyclic aromatic hydrocarbon Heterocyclic organic compounds Phthalate esters Benzophenones Ketones Fatty amides

121

Carbonized (850 °C)

+ + + + + + + + + + + + + + + + + + + + +

Raw sample + +

+ + +

+ + + + + +

+ + +

b

Main m/z 43,58,27,41 91,106,105,77 43,58,59,71 43,71,41,55 106,105,77,51 105,120,119,77 43,44,41,29 117,118,115,91 99,44,42,98 43,58,41,57 105,77,120,51 57,43,71,41,85,29,56 108,91,43,90 128,129,127,51 41,29,55,84 57,55,43,41 43,57,41,85,71,29,56 43,58,71,41 142,141,115,143 141,142,115,139 104,76,148,50 154,153,152,155 57,43,71,41 170,141,77,51 156,141,155,115 206,208,127,63 55,74,87,143 168,139,169,84 149,177,150,105 105,77,182,51 58,43,59,71 59,72,43,41

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curves was very good. The results in Fig. 5 a-c also showed that the differences between calculation results and experimental results were very small. Generally speaking, there is no great influence of the heating rate on the values of kinetic parameters of pseudo-components decomposition. This is particularly true for the main lignocellulosic components. In addition, the obtained pair of kinetic parameters (A and Ea) for the main lignocellulosic components (cellulose, hemicelluloses and lignin) is lower from those identified for the typical wood biomass or for the individual (pure) components [48]. Meanwhile, the values of Ea were related to the shape of the peaks. The broader the peak was, the lower the apparent activation energy was. This fact was specifically identified for all pseudo-components in the case of Poplar fluff pyrolysis. The kinetic parameters for lignin decomposition are comparable with generally reported in literature (20–65 kJ mol−1) [49]. The low apparent activation energy (Ea) for lignin is reflected by the complex structure and thus broad temperature range under which the decomposition takes place. In connection with the above facts, it should be noted that the apparent activation energy can reflects to a large extent the contribution of cellulose during pyrolysis process of biomass sample, and does not correspond to the energy barrier for any concrete reactions occurring during Poplar fluff pyrolysis. Namely, between 200 and 400 °C (Fig. 5), the processes of depolymerization, decomposition and re-arrangement occur, and these include a considerable mass losses (Fig. 3 a). Chemical bonds of the starting material break (the primary reaction) generating a residue (bio-char), plus the liquid (tar) and gaseous compounds, taking place by different mechanisms. However, in the presence of metal catalysts, in respect to realizing the specific type of the reactions attached to biomass pseudo-components can lowered the activation parameters such as Ea and A. Therefore all these facts were further explained in more detail. It should be noted that lower values of the kinetic parameters for hemicelluloses and cellulose decomposition reactions in the Poplar fluff pyrolysis (especially expressed through φ and A values) can be a consequence of catalytic activities of the mineral content. This phenomenon affects on the merging into one single DTG peak (Fig. 3 b), and almost very little difference in dispersion of deconvoluted peaks in Fig. 5 a-c, for these pseudo-components (ΔσPSC-1 = 37.28 °C (cellulose) and ΔσPSC-2 = 36.20 °C (hemicelluloses) over all heating rates). In addition, the extractives devolatilization was characterized by the apparent activation energy values in the range of 57.1–107.6 kJ mol−1 (Table 3) which are in agreement with reported literature values [50,51], and depends on the extractives content in the investigated biomass sample. All estimated kinetic parameters by the application of independent parallel reactions model for Poplar fluff pyrolysis are higher than those founded for the fractionated cottonwood pyrolysis [52], but lower than those obtained for Cotton seed pyrolysis [53]. The obtained values of kinetic parameters for Poplar fluff pyrolysis are between the values obtained for these two last cases. Any potential differences in the values can be a consequence of the specific experimental conditions applied in monitoring the pyrolytic process (such as sample sizes, the heating rates applied, calculation procedures, sample holder configurations, methods of heating, etc).

Fig. 6. The FTIR spectra of (a) native (raw) and (b) carbonized (850 °C) Poplar fluff samples.

Fig. 6 and they are in compliance with the results obtained by chemical composition analysis (Table 2), which provides that carbohydrates (cellulose, hemicelluloses and lignin), and extractives represent the most of the chemical composition. As can be seen from Fig. 6, the raw sample shows broad band at 3413 cm−1, which corresponds to absorption by hydroxyl group in polysaccharides structure (cellulose, hemicelluloses and lignin) [54]. Two bands at 2851 and 2929 cm−1, observed in the FTIR spectra of the raw sample are attributed to deformation vibrations of CeH groups in methyl and methylene groups belonging to cellulose and hemicelluloses structures [55], as well as to lignin [54]. The band with maxima at 1743 cm−1 originates from hemicelluloses and lignin ester C]O stretching vibration [56]. The absorption peak at 1643 cm−1 originates from lignin aromatic skeletal vibration [57], and it is overlapped with a peak that originates from water molecules adsorbed at the hydroxyl groups present in lignin, cellulose, and hemicelluloses. The lignin aromatic skeletal vibration can also be observed at 1510 cm−1 [57]. The bands at 1326 and 1247 cm−1, originate from the lignin CeO vibration, while bands at 1158 and 1113 cm−1 originate from CeOeC asymmetrical stretching vibration in cellulose/hemicelluloses and lignin (the presence of both, syringyl and guaiacyl subunits) structure. The CeO valence vibrations can be observed at 1050 up to 1015 cm−1 [58]. Vibration at about 1050 cm−1 may be originated by bending vibrations from alcohols involved in weak hydrogen bonding. Liu et al. [59] describe bands in the region of 956 – 1050 cm−1 as C—O vibrational peaks from cellulosic alcohols in cotton fibers material. After carbonization process, the lignin/cellulose hydroxyl group is associated with the peak at 3413 cm−1, which was probably caused by the decomposition of phenolic hydroxyls. During thermal treatment, the dehydration reaction of aliphatic hydroxyl groups linked to ß- and γ-carbon atoms may leads to the compounds which possess an alkyl or the vinyl groups. This can lead to formation of monomeric pyrolyzates with aromatic structures. Namely, if we assume that pyrolyzates are mostly monomeric initially and oligomerized through reaction with each other, the oligomerization reaction would be much less favored in bio-char-entrapped molecules, because of the competition with absorption to the bio-char surface. The dimerization may still happen, but the reaction probability for trimerization would be in that case very low. The observed broad, low intensity peak, around 1567 cm−1originates from C]CeO vibration and its low intensity is probably caused by the new formed groups related to lignin/cellulose backbone breaking [60]. The lignin undergoes a variety of thermal modification and decomposition stages, such as dehydration, cleavage of ether linkages, and hydrogenation of alkenes, the elimination of carbonyl entities, demethylation and demethoxylation. At the high

4.4. Comparative analysis of the characterization of raw precursor and carbonized material 4.4.1. FTIR results The elemental and functional group composition of the raw and carbonized (850 °C) Poplar fluff sample was analyzed using Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy. The distribution of elements and functional groups in the studied sample is vital for determining the composition and distribution of pyrolysis products. Therefore, the comparative compositional analysis of native (raw precursor) and carbonized Poplar fluff samples using the FTIR spectroscopy was performed. The corresponding FTIR spectra are shown in 122

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terms of combustion behavior of resultant fuel. This gives that the fuel is more energy dense, easily friable, and allows overcoming the issues related to slagging and fouling, associated with starting biomass feedstock. 4.4.3. GC–MS results of bio-oil characterization The biomass pyrolysis vapors consist of volatile compounds and non-volatile oligomers. GC–MS was only able to determine the volatile organic compounds. In this research, the ion chromatogram was obtained from the pyrolysis of the Poplar fluff. A total of 32 major compounds were identified as given in Table 4. The GC–MS chromatograms of the bio-oils are illustrated in Figs. 8 and 9. The compounds identified in bio-oils are also listed in Table 4. As shown in Table 4, the bio-oil obtained from the raw material was mainly composed of fatty amides including hexadecanamide, polycyclic aromatic hydrocarbons including naphthalene, naphthalene, 1-methyl, pyrrolidines with 2-Pyrrolidinone, 1-methyl, as well as aldehydes, acids, esters and ketones. The fatty amides (such as hexadecanamide) originate from the lipids presented in raw Poplar fluff sample, where also polycyclic aromatic hydrocarbons (PAH’s) occur, but with lower abundance. So, the lower temperatures can be suggested as a more suitable for larger application of raw material to avoid the merged formation of PAH’s. The identified occurrence of pyrrolidines may probably originate from metal catalyst reductive amination, which occurs through derived acids by the biomass carbohydrates decomposition with a presence of H2 source. Acids dominantly originate from the hemicelluloses decomposition, while the cellulose decomposes to ketones and aldehydes [63–65]. Taking into account the presence of esters in the case of raw material, the evolutions of CO and CO2 during the pyrolysis of Poplar fluff originate from the decomposition of ester group of the molecule in the chain propagation reactions. The CO can be obtained from decomposition of C11H21O2 radicals with probable radical center located in positions “4” and “6”. Probably these radicals decompose by the successive reactions of ß-scission leading to C3H5O2 radical. However, the reaction of ß-scission leads to formation of ketones and this can be disfavored by a high activation energy values and this radical mainly isomerizes by the shifting an hydrogen atom from methyl group of the ester function to the methylene group. Then, probably, a new radical leads to CO formation through the two successive reactions of ß-scission. Medium-chain aldehydes can be formed by the radical-chain addition followed by the reactions of decomposition. It should be emphasized that this proposed reaction scheme is in very good agreement with the above-mentioned results and in particular, with the comprehensive values Ea and A listed in Table 3. The carbonized Poplar fluff exhibits a great abundance of polycyclic aromatic hydrocarbons (PAH’s), heterocyclic compounds, and aromatics as well as aromatic derivatives (Table 4), favored at the high temperatures. This can be clearly seen in Fig. 8 by the major peak representations, such as 14, 19 and 20 at retention times 12.53, 14.52 and 14.82, respectively. So, the obtained solid product (bio-char) contains the large amounts of non-volatile compounds, with a fairly large share of PAH’s. The GC–MS has proved to be an excellent identification tool technique for PAH’s as pyrogenics in the sense of the charred Poplar fluff. So, the presented results of GC–MS analysis of carbonized material unambiguously shows that it is an important source of organic compounds containing the carbons and hydrogen’s composed of multiple rings. The identified compounds are useful for making medicines, plastics, dyes, and pesticides. Concerning the PAH species in carbonized process case, the dominant compounds are naphthalene (NAP) and naphthalene, 1-methyl (NAP-1-m) (Fig. 8), where these PAH are most abundant. It can be emphasized that the level of toxic polycyclic aromatics must be strictly regulated, so, it is well-known that bio-char with lower levels of PAH is considered to be quality. In our case, the PAH’s with C10 (two-rings) and C12 (three-rings) (less-condensed C structure) dominate, where the increased carbonization temperatures may result in the higher fractions of stable C and total C due to an increased release

Fig. 7. XRD pattern of carbonized Poplar fluff sample at 850 °C.

carbonization temperatures, the lignin decomposition products may further undergo thermal modification of methoxyl groups to form hydroxyl groups, simultaneous with demethoxylation-to-alkylation reaction and positional re-arrangement of methoxyl groups. 4.4.2. XRD results of carbonized material In first approximation, the shape of XRD diagram provides us an idea about the order/disorder degree of the structure of carbonized material. The XRD pattern of the carbonized Poplar fluff sample at 850 °C is presented in Fig. 7. The appearance of two broad peaks with different intensities around 25° (marked by “θ1”) and 43.1° (marked by “θ2”) are observed in carbonized sample. These peaks confirm the presence of graphite structures in the studied material. The peak situated at 25 °C can be related to (0 0 2) graphite structure, which may be indication for the stacking of a few graphene-like layers into the obtained carbons [61]. The peak at 25 °C is typical for commercial bulk reactor graphite single crystal. The peak at 43.1° corresponds to (1 0 0) plane of the carbon crystals. In this respect, both intense peaks in XRD pattern of the carbonized sample are strong indication of the metal catalyzed carbons. The increased intensity of the first in relation to the second peak can be correlated with mineralogical phase of graphite (Table S2, Supplementary material). A significantly higher intensity of the first peak can be supported by the acceleration of the development of atomic order and crystallite sizes (crystalline structure with improved layer alignment) with the mineral compounds including silicon, or even iron compounds (Table S2). Namely, the process of catalytic graphitization can be significantly accelerated at even lower temperatures in the presence of catalysts such as Fe (Fe – catalyst for graphitization promotion). The magnesium (Mg) is absent in the carbonized Poplar fluff sample (Table S2). The high carbonization temperature leads to removal of Mg and Cu residuals from the bio-char (Supplementary material). However, the increasing of carbonization temperature (actually 850 °C), promotes the release of Al (aluminum) (Table S2, Supplementary material) which can acts (together with heat treatment) as pulsar in accelerator processing from transformation of carbon into graphite [62]. A removal of alkali metals is evident, and this can be a turning-point in ash chemistry for combustible properties. A number of predictive slagging and fouling indices have been used to evaluate the influence of ash chemistry on Poplar fluff combustion behavior, and this can be combined with ash fusion testing, which would probably shows the reduced potential of fouling and slagging in resulting bio-char if it combust. However, the limited removal in the iron (Fe) concentration in carbonized sample in comparison with initial (raw) sample (Supplementary material) suggests on its accumulation within bio-char, resulting in improvements in bed agglomeration index and slag viscosity index. Removal of magnesium has a significant influence on slagging and fouling propensity of Poplar fluff as a fuel, which demonstrates the importance of metal removal in 123

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Fig. 8. GC–MS chromatogram of bio-oil of the carbonized material at 850 °C.

of volatiles. In particular, the bio-char produced at high carbonization temperature has high aromatic content, which is recalcitrant to the decomposition. Results also show that the higher carbonization temperature has the potential of accumulating heavy metals (see Supplementary material), which can cause the soil pollution. Therefore, it is not recommended for soil amendments. So, the bio-char obtained at higher temperatures possesses predominately aromatic carbon structures and higher thermal stability, and in actual case, it can be useful to help the mitigate climate change. However, the amount of heavy metals and PAH graduation

must be balanced, between the duration of the carbonization process and applied temperature. Namely, it was reported [66] that with an increasing of the pyrolysis time and the temperature, the PAH concentrations generally decreased, but Ledesma et al. [67] showed that the large contents of PAH are produced at temperature greater than 700 °C. The PAH’s formation has various mechanisms, and wherein the hydrogen abstraction acetylene addition (i.e. HACA), is widely acknowledged [68]. The naphthalene (Table 4) could be generated from benzene through intermediate phenylacetylene, which usually was

Fig. 9. GC–MS chromatogram of bio-oil of the raw material. 124

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Fig. 10. Scanning electron microscopy (SEM) micrographs of the bio-char sample carbonized at 850 °C. The SEM images with different magnifications: a) 1000×, b) 2000 × and c) 2000×, d-h) 5000 × .

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abundant in the tar. So, the single ring compounds (such as benzene) could be regarded as PAH’s precursors. For components with aromatic rings, such as lignin, PAH’s might be formed from the aromatic rings through HACA mechanism [68,69]. On the other hand, at the high temperature, the polysaccharides (hemicelluloses and cellulose) could generate PAH’s through two mechanisms: Diels-Alder reaction and deoxygenation of oxygenated aromatics [70]. Since the concentration of alkenes is very low (Fig. 8) in a gas phase, Diels-Alder was unlikely to happen. Therefore, it is more likely that PAH’s might be generated from polysaccharides through benzene as intermediate. In addition, naphthalene had a different grow routes, which may leads to benzenoid PAH’s or cyclopentaring–fused PAH’s. Still, there is another possible mechanism of benzenoid PAH’s formation. The benzenoid PAH’s might be formed from interactions of cyclopentadiene – indane (Table 4). However, the carbonized Poplar fluff generates the large amount of naphthalene, which means that HACA process is probably happened. It should be noted that with an increasing of the temperature above 800 °C, the tar molecular weight probably increased. The possible cause for this would be the fact that the secondary reactions of PAH’s growth can be promoted with the increase of the temperature. It should be emphasized that the PAH’s generation in pyrolyzed raw Poplar fluff sample may originates from the presence of copper (Cu) (Supplementary material), where, for example, copper (II)–chloride can influenced on the PAH’s productions [71]. It should be noted that the major pyrolysis compounds from lignocellulosic bio-char consists mainly the aromatic and heterocyclic compounds, where some of them come from incomplete charred lignocellulosic material (Table 4). The phenolic compounds of pyrolyzed Poplar fluff are the major components of tar. So, the major constituents of tar at high temperature are the following: PAH’s, indanes, benzene and substituted derivatives (Table 4). On the basis of the obtained results, it can be assumed that the condensation reactions and subsequent transformations of platform chemicals (such as aldehydes and ketones) provide facile approaches to construct larger products with structures, functionalities, and physical properties that are desirable for use as transportation fuels or specialty chemicals. At higher temperatures, catalytic condensation reactions may occurs yielding a more complex reaction pathways and produces acyclic, cyclic and aromatic chemical entities. In addition, it can be noted that diphenyl ethers (Table 4 and Fig. 8) are formed by the lignin pyrolysis that occurs at higher temperatures, and the higher than fast decomposition of cellulose. Likewise, even small quantities of alkaline cations would shift the pyrolytic pathways of cellulose and can promote the formation of ring-scission products and bio-char on the expense of levoglucosan [72].

analysis suggests that the obtained bio-char has heterogeneous structure consisting of the amorphous phase and the graphite crystalline phase. In addition, the amorphous phase was generated along with pyrolysis volatilization of Poplar fluff, and was dominant inside the biochar, which provides the basic porous structure and foundation for the pore structure development via further activation or some modifications. This was in good agreement with XRD analysis (Fig. 7) that graphitization occurred during the carbonization. 5. Conclusions In this work, the Poplar fluff (Populus alba) was pyrolyzed to convert into gaseous, liquid and solid (bio-char) products using on-line pyrolytic device for identification of devolatilization products and the fixed bed reactor at static carbonization temperature. The chemical properties of obtained products were characterized. In addition to the characterization, the kinetic analysis of the devolatilization process was carried out using the multi-component model. It was found that Poplar fluff has higher energy content since on increased H/C ratio, which is associated with increased content of fibrous structure, specially cellulose. Founded lower atomic ratio O/C and higher atomic H/C ratio in a raw material were suggested on aliphatic carbon containing compounds that can decrease, and increasing of the aromatic compounds. A significant amount of the ash producing by Poplar fluff represents the notable lack of studied sample, taking its application for heat boilers. The estimated HHV value (15.86 MJ kg−1) for Poplar fluff corresponds to HHV’s of agricultural residue biomasses, and within the range of HHV’s that fit those values for lignite/brown coal (fossil) fuel. It was found that the Poplar fluff by the chemical composition is most similar with Cotton seed. It was found that high Mg-magnesium ash content can play an important role in catalytic process in high temperature producing of bio-char and especially with reactions with CO2. It was established that high ash content provides a larger, potentially catalytically active surface on which secondary tar polymerization and bio-char forming reactions can take place. Complete characteristics of the thermal stability of studied sample under pyrolytic conditions were explained where the minimum thermal-lag effects were identified. It was found that presence of metal catalysts, in respect to realizing the specific type of decomposition reactions attached to pseudo-components can lower activation parameters. The bio-oil derived from Poplar fluff consist about 32 major compounds, including fatty amides, aldehydes, acids, esters and ketones. It was found that carbonized Poplar fluff sample at 850 °C exhibits a great abundance of polycyclic aromatic hydrocarbons (PAH’s). Within identified PAH species, the naphthalene (NAP) and naphthalene, 1-methyl (NAP-1-m) are the most abundant. Mechanism for PAH’s formation in the case of carbonized Poplar fluff was proposed. Results pointed that the hydrogen abstraction acetylene addition (HACA) probably occurs. It was assumed that PAH’s can be generated from polysaccharides through benzene as intermediate. Also, it was proposed that PAH’s generation in pyrolyzed Poplar fluff sample may originates from the presence of copper, which can influenced on PAH’s production. Bio-char obtained in the high-temperature charring process (850 °C) showed heterogeneous structure consisting of amorphous phase and graphite crystalline phase. It was found that the graphitization occurs during carbonization, which was confirmed by XRD analysis. Obtained bio-char has characteristics that would enable it the effective porosity with improved surface area, such as, for the electrochemical energy storage (Poplar fluff-derived renewable carbon material).

4.4.4. Surface morphology of the obtained bio-char by carbonization process using SEM analysis Fig. 10 shows the SEM micrographs with various magnifications (1000×, 2000× and 5000×) of bio-char produced at 850 °C. The carbonized sample shows the hollow micro-tubular structure with the thin layers (Fig. 10 a-c). The high magnification SEM image shows the existence of the fibers with averaged diameter around 10 μm (Fig. 10). However, the retaining of original fiber structures (from starting (raw) material) in bio-char was evident (Fig S1 a – b presented in Supplementary material). It can be observed that the fiber structure continues (with ampled of free space which was observed between the microfiber bundles (Fig. 10 h)), with the surface exhibiting roughly, but porous surface morphology, with pronounced cracks and wide holes at the ends of fibrillating tubes (see Fig. 10 d – g). In comparison with raw material (Fig S1 a – b (Supplementary material)), the bio-char morphology exhibits much more orderly and clearly focused bundles of fibrillous structures, without significant twisting. Additionally some irregular and discontinuous slits could also be observed, which can contributes to an increased adsorption capacity of carbonized material. The morphology

Acknowledgments Authors would like to acknowledge financial support of Ministry of Education, Science and Technological Development of the Republic of Serbia under the Projects 172015, III42010, III45005 and III 45001. 126

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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2018.10.064.

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