Reaction schemes for estimating kinetic parameters of thermal decomposition of native and metal-loaded almond shell

Accepted Manuscript Title: Reaction schemes for estimating kinetic parameters of thermal decomposition of native and metal-loaded almond shell Authors: L. Quesada, A. P´erez, M. Calero, G. Bl´azquez, M.A. Mart´ın-Lara PII: DOI: Reference:

S0957-5820(18)30430-0 https://doi.org/10.1016/j.psep.2018.06.041 PSEP 1444

To appear in:

Process Safety and Environment Protection

Received date: Revised date: Accepted date:

26-2-2018 20-6-2018 29-6-2018

Please cite this article as: Quesada, L., P´erez, A., Calero, M., Bl´azquez, G., Mart´ın-Lara, M.A., Reaction schemes for estimating kinetic parameters of thermal decomposition of native and metal-loaded almond shell.Process Safety and Environment Protection https://doi.org/10.1016/j.psep.2018.06.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Reaction

schemes

for

estimating

kinetic

parameters

of

thermal

decomposition of native and metal-loaded almond shell

Quesada, L.; Pérez, A.*; Calero, M.; Blázquez, G.; Martín-Lara, M.A. *Corresponding author

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Lucía Quesada Department of Chemical Engineering University of Granada, 18071 Granada (Spain) Phone: 34 958 244075

e-mail: [email protected]

Antonio Pérez Department of Chemical Engineering University of Granada, 18071 Granada (Spain) Phone: 34 958 244075

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Fax: 34 958 248992

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e-mail: [email protected]

Mónica Calero de Hoces

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Department of Chemical Engineering University of Granada, 18071 Granada (Spain) Phone: 34 958 243315

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Fax: 34 958 248992 e-mail: [email protected]

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Gabriel Blázquez García

Department of Chemical Engineering University of Granada, 18071 Granada (Spain) Phone: 34 958 243311 Fax: 34 958 248992

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e-mail: [email protected]

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Fax: 34 958 248992

María Ángeles Martín-Lara

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Department of Chemical Engineering University of Granada, 18071 Granada (Spain) Phone: 34 958 240445 Fax: 34 958 248992

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e-mail: [email protected]

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Graphical abstract

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HIGHLIGHTS

Isolation of chemical constituents of almond shell was completed.

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Study of thermal decomposition different fractions of almond shell was performed.

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A kinetic model for describing thermal decomposition of almond shell was

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•

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proposed.

The experimental and calculated data were in good agreement.

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The effect of various heavy metal loaded by biosorption was analyzed.

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Abstract

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•

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This research aims to provide a better knowledge of the thermal decomposition of almond shell and this material loaded with heavy metals in a previous stage of

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biosorption (cadmium, copper, chromium, nickel and lead). Firstly, isolation of constituents of the almond shell was carried out. According to chemical analysis almond shell was constituted of an 8.2% of moisture; 1.6% of hot water soluble compounds; 0.34% of ethanol soluble compounds; 50.8 % of extract-free lignin; and 49.2% of extract-free holocellulose. Then, experiments were performed by thermogravimetric

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analysis (TGA) and differential thermogravimetry (DTG) under inert and air atmosphere at a heating rate of 15 K/min for each isolated fraction. After that, adequate reactions schemes were proposed to find kinetic parameters. Independent reactions were formulated for each constituent and kinetic parameters were obtained for each isolated material in a sequential procedure. Finally, the validation of the proposed schemes was

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verified by the goodness of fitting between experimental and simulated TG and DTG curves.

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It was observed that cadmium, copper, chromium, nickel and lead present in metalloaded almond shell did not modify significantly the values of kinetic parameters which

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describe the thermal decomposition processes.

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Keywords: Almond shell; Biosorption; Heavy metals; Kinetics; Reaction schemes;

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Thermal decomposition.

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1. INTRODUCTION

Currently the conventional energetic model is based on non-renewable energy

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resources, namely petroleum, coal and natural gas. Limitation of fossil fuel resources, steady increase in the prices of oil crude, as well as the increasing environmental

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concern about greenhouse gas emissions are some problems related with the current energetic model. An alternative solution for these problems is the use of renewable energies such as biomass, solar, wind, geothermal, and hydropower. However, among

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all these technologies, one of the most important is biomass, which is expected to become the renewable energy with the higher growth. According to estimation of the International Energy Agency, biomass will achieve a 10% of use as an energy source at 2030 (IEA, 2015).

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Within the different types of biomass materials, agricultural waste brings the greatest potential. The agricultural residues are usually lignocellulosic materials composed basically of hemicellulose, cellulose and lignin. This composition indicates that they could be used as fuels and for the production of sugars and other added value products. Besides, the use of lignocellulosic biomass to produce heat and power is

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environmentally beneficial because biomass is a renewable resource and the combustion of biomass does not contribute to the emission of greenhouse gases into the atmosphere

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(López et al., 2010; Feria et al., 2011; Ma et al., 2017). Therefore, problems related with the current energetic model will be reduced, and the residues will be managed efficiently.

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Among all agricultural residues, almond shell has a high potential to be used as fuel. In

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fact, the almond plantation area in Spain was 5,270 km2 in 2014 and a total production

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of 196,000 tons of almond was reported. In Andalusia, southern Spain, the production

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of almond is increasingly standing out, so there are little initiatives using almond shell

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as a source of biomass for energy production. In recent years, almond shell has been used as fuel for industrial furnaces, ceramics and

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heating systems for livestock farms and nowadays is also used for stoves and domestic boilers. Even though the calorific value of the almond shell is about half of the calorific

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value of diesel, it is important to remark that the price of diesel is around 0.80 €/ liter (2014-2015), while the almond shell has an average price of around 0.135 € / kg (2014-

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2015) (IEA, 2015). Lastly, it should be noted that the generation of energy from almond shell is an emerging and promising alternative. In this sense, some recent works were published about using of almond shell as fuel (García et al., 2017; Chiou et al., 2018; Mediara et al., 2018). Moreover, the use of almond shell as fuel seems to be an effective option for

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reducing carbon dioxide emissions up to 50 million tons by employing 32,000 tons of almond shell briquettes. This quantity of briquettes is able to provide the thermal energy in more than 20,000 homes, thereby, curbing utilization of 15,000 tons of oil (IEA, 2015). On the other hand, investigations about heavy metal removal by using almond shell as

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biosorbent has have been previously published (Ronda et al., 2013; Khan et al., 2015;

Banerjee et al., 2017; Maaloul et al., 2017; Yildiz, 2017; Banerjee et al., 2018; Cataldo

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et al., 2018; Taha et al., 2018). Most studies have focus on biosorption capacity of the proposed biosorbent whereas some of them have studied regeneration processes. However, very few authors have studied the final disposal of the exhausted biosorbent

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required in industrial applications (Pehlivan and Altun, 2008; Pehlivan et al., 2009;

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Martín-Lara et al., 2016; Almendros et al., 2017; Martín-Lara et al., 2018a).

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In this work, the kinetics of the thermal decomposition of almond shell and its different

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solid fractions obtained by chemical treatment was studied. Adequate kinetic models

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that predict their potential for the production of bioenergy in an inert atmosphere of nitrogen and in an atmosphere of air were proposed. Also, the effect of the heavy metals

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(previously loaded by biosorption) on kinetics was studied. With regard to previous published papers about thermal decomposition of metal-loaded

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almond shell, only Martín-Lara et al., (2018b) subjected heavy metal-loaded almond shell to pyrolysis to study the effect of the presence of different heavy metals on its

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thermal degradation. Nevertheless, this study focused more on volatile and semivolatile emissions than on kinetics of the thermal decomposition and not presents a study of thermal degradation in an oxygen environment.

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2. MATERIALS AND METHODS 2.1. Raw material The almond shell (AS) was purchased from a biomass company of Granada (Spain). Firstly, AS was milled and sieved to obtain a particle size smaller than 1.4 mm. Then, the sample was fractionated into its organic fractions according to procedure indicated

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in sub-section 2.4. 2.2. Physic-chemical characterization of AS

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2.2.1. Proximate analysis

The moisture content of AS was determined by the difference in weight between the wet

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sample and the sample after drying in an oven (at 378K) until constant weight according

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to standard method ISO 18134-2:2017.

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Volatile matter content was calculated according to standard ISO 18123:2015. A sample

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was placed in airtight vessel and heated up to 1173 K during 7 min. The percentage of volatile matter was calculated from the mass loss of the sample excluding the mass loss

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due to moisture.

Ash content was determined by measuring the weight of residue after calcination at 823

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K during 60 min on air atmosphere, according to standard ISO 18122:2015.

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Fixed carbon content was calculated by difference until 100%. 2.2.2. Chemical composition of AS Firstly, the water-soluble fraction was evaluated according to TAPPI T 257 standard.

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Then, ethanol–benzene soluble compounds were determined according to TAPPIT 204 standard. Finally, lignin and holocellulose were determined according to TAPPI T 222 and Wise method, respectively (Wise et al., 1946; Tappi, 2017). 2.2.3. Elemental analysis

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Elemental analysis of dried AS sample was accomplished by combustion analysis using an Elemental Fison’s Instruments EA1108 CHNS. 2.2.4 Calorific value The calorific value was performed with a calorimetric pump (Phywe LEC-02) according to the standard method EN 14918:2009.

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2.3. Preparation of metal–loaded AS samples

The impregnation of AS with heavy metals was carried out by biosorption. The

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biosorption tests were performed in a batch reactor equipped with magnetic stirring. In

each test, 20 g of biosorbent were kept in contact with 2 L of heavy metal solution with

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a concentration of 200 mg/L of heavy metal. The tests were performed at constant

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temperature of 298 K, until equilibrium was reached (approximately 120 min).

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The metal salts used to prepare the metal solutions were nitrate salts of different heavy

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metals (Cd (II), Cr (III), Cu (II), Ni (II) and Pb (II)), all of them purchased from Panreac, S.A. (Spain).

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Once equilibrium was reached, the samples were centrifuged and filtrated. Solid phase was washed with deionized water until the non-bounded ions were removed. Then, the

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solid was dried in an oven at 313 K during 24 h. Liquid phase was analyzed to

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determine metal concentration by atomic absorption spectrophotometry using a PerkinElmer AAnalyst 200 spectrophotometer. Determination of the amount of metal retained in solid (mg of metal / g of solid), q, was

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determined according to the following mass balance: 𝑞 = (𝐶𝑖-𝐶𝑓)·𝑚

(Eq. 1)

Where Ci and Cf are the initial and final concentrations of metal in the solution (mg/L) and m is the ratio between volume of metal solution and mass of solid (L/g). 2.4. Isolation of constituents of AS by chemical fractionation

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The main compounds of the AS were isolated in order to study the kinetics of the pyrolysis and combustion of each component and to propose the most adequate kinetic models that predict the potential of the solid for the production of bioenergy. The chemical fractionation was performed by several consecutive steps. A more detailed procedure was previously published by Pérez et al., 2018 and Ronda et al., 2017.

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First step: Extraction of hot water-soluble compounds.

Approximately 1 kg of sample was mixed with 5 L of hot water (353 K). The mixture

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was stirred during 3 h, and the temperature was raised up to 373 K. Then, the solution was filtered in a calibrated plate number 2 with hot water. Finally, the sample was dried for 24 h.

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Second step: Extraction of ethanol-acetone soluble compounds.

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A sample of 20 g was placed in a filter paper cartridge and it was introduced in a

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Soxhlet extractor with ground glass mouth Erlenmeyer flask. A 1:2 ratio mix of ethanol-

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acetone was added to perform extractions. This step was finished when mixture of

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solvent was not colored in the siphon.

The flask with the extracted sample was dried until the weight does not vary, thus, the

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difference between weights was the extracts content. The cartridge with the sample was also dried and weighed to obtain the yield of process.

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Third step: Acid extraction.

The acid fraction content extraction and acid hydrolysis was performed using

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concentrated H2SO4, 2 g of the solid sample from second step was placed in a beaker, then, 30 mL of concentrate H2SO4 (72%) was added. The mixture was storage during 2 hours, later; it was transferred to another flask. Distilled water was used to fill up to 600 mL. Then, the sample was heated under reflux condition during 4 hours. Finally, the sample was filtered and it was dried for 24 h.

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Fourth step: Oxidative extraction. A sample of 5 g of the solid from the second step was placed in an Erlenmeyer flask and 160 mL of water was added. Then, the mixture was heated up to 348-353K in a thermal constant bath. Later, a weight of 1.5 g of NaClO2 and a volume of 0.5 mL of glacial acetic acid were added.

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The sample was stirred until coloration was observed. Finally, it was cooled and filtered in a calibrated plate of number 2. Finally, the sample was dried during 24 h.

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2.5. Pyrolysis experiments and model formulation

Pyrolysis tests were carried out on a Perkin Elmer thermobalance model STA 6000. Dynamic experiments were performed with a mass of approximately 30-50 mg of

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sample heated from 416 K to 1073 K at a heating rate of 15 K/min. Nitrogen (high

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purity) was used as carrier gas with a flow rate of 20 mL/min during the entire pyrolysis

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tests. All trials were repeated at least three times and the average value was used for

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calculations.

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The kinetic model for pyrolysis was related to AS chemical composition, in order to set up a model useful for this material (Molto, 2007). Hence, it was considered that AS was

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formed by three independent fractions: i) hemicellulose, ii) cellulose, and iii) lignin, which exhibit great differences that can be related to their chemical structure. Therefore,

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it can be considered that each fraction follows independent reactions. These reactions can be expressed as follows: Solidi → ʋivolatilei + ci carbonaceous solidi

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(Reaction 1)

Where solidi refers to each constituent of AS (i=1 hemicellulose, i=2 cellulose and i=3 lignin) that decomposes to generate a solid residue (char and ash), called carbonaceous solidi and volatiles (gases + volatiles), called volatilei. Also, ʋi is referred to the yield

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coefficient of volatiles and ci is referred to the yield coefficient of carbonaceous solid. These last coefficients are related by the following equation 𝑐𝑖 = 1 − 𝜈𝑖

(Eq. 2)

The conversion degree is then introduced to facilitate the model description, as follows: ∝𝑖 =

𝑉𝑖

(Eq. 3)

𝜐𝑖

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Where Vi is the mass fraction of volatiles obtained at any time by reaction n.

The proposed model also considers that all the possible interactions among each

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component have a negligible effect on the pyrolysis process. Therefore, the kinetic

equation for each independent reaction is defined as follows (defined as nth order

𝑑𝑡

=𝑘𝑖1 (1 − 𝛼𝑖1 )𝑛𝑖1

(Eq. 4)

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𝑑𝛼𝑖1

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reaction),

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It is observed that the kinetic constants are independent of the composition of each

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component in the native solid. Moreover, they follow the Arrhenius equation. Therefore, equation 3 can be rewritten as: 𝑑𝑡

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𝑖1 =𝑘𝑖10 ∙ 𝑒𝑥𝑝 (− 𝑅∙𝑇 ) ∙ (1 − 𝛼𝑖1 )𝑛𝑖1

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𝑑𝛼𝑖1

(Eq. 5)

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Finally, the total volatile fraction (Vtotal) and the total solid fraction (wcal) are related to other variables by the following equations: (Eq. 6)

wcal =1- Vtotal

(Eq. 7)

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𝑉𝑡𝑜𝑡𝑎𝑙 = ∑31 𝜐𝑖1 ∙∝𝑖1

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The model variables were obtained using the Solver function of Microsoft Excel by minimizing the following objective function (FO): 𝐹𝑂 = ∑(𝑤𝑗𝑒𝑥𝑝 − 𝑤𝑗𝑐𝑎𝑙 ) + ∑ (

𝑒𝑥𝑝

𝑑𝑤𝑗

𝑑𝑡

−

𝑑𝑤𝑗𝑐𝑎𝑙 𝑑𝑡

)

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(Eq. 8)

2.6. Combustion experiments and model formulation Combustion tests were also carried out in a Perkin Elmer thermobalance model STA 6000. Dynamic experiments were performed with a mass of approximately 30-50 mg of sample heated from 416 K to 1073 K at a heating rate of 15 K/min. Synthetic air (79% nitrogen and 21% oxygen) was used as carrier gas with a flow rate of 20 mL/min during

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the entire combustion tests. All trials were repeated at least three times and the average value was used for calculations.

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The proposed kinetic model for the combustion process was based on the pyrolysis one, in which new reactions between the oxygen and each fraction were introduced.

Solidi → ʋi1 volatilei1+ ci1 carbonaceous solidi1

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Carbonaceous solidi2 + O2 → ʋi4 volatilei4

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Carbonaceous solidi1 + O2 → ʋi3 volatilei3

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Solidi + O2 → ʋi2 volatilei2 + ci2 carbonaceous solidi2

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The reactions can be summarized as follows:

(Reaction 1) (Reaction 2) (Reaction 3) (Reaction 4)

be written as follows: 𝑑𝑡 𝑑𝛼𝑖2

(Eq. 9)

=𝑘𝑖2 (1 − 𝛼𝑖1 − 𝛼𝑖2 )𝑛𝑖2

(Eq. 10)

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𝑑𝑡

=𝑘𝑖1 (1 − 𝛼𝑖1 − 𝛼𝑖2 )𝑛𝑖1

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𝑑𝛼𝑖1

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Now, the reactions 1 and 2 are competitive with respect to solidi and the kinetic law can

Also, the kinetics of the combustion of carbonaceous solids (reactions 3 and 4) is given by the following equations:

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𝑑𝛼𝑖3 𝑑𝑡

𝑑𝛼𝑖4 𝑑𝑡

=𝑘𝑖3 (1 − 𝛼𝑖1 − 𝛼𝑖3 )𝑛𝑖3

(Eq. 11)

=𝑘𝑖4 (1 − 𝛼𝑖2 − 𝛼𝑖4 )𝑛𝑖4

(Eq. 12)

Also, in reactions 2, 3 and 4, which are the reactions of combustion, the variation of ki is given by the following expression

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ki = ki0(PO2)biexp (-Ei/RT)

(Eq. 13)

Where PO2 represents the oxygen partial pressure (0.2 atm. in all tests) and bi is the oxygen reaction order. Finally, the total volatile fraction and the total solid fraction are related to other variables by the following equations: 𝑉𝑡𝑜𝑡𝑎𝑙 = ∑31(𝜐𝑖1 ∙∝𝑖1 + 𝜐𝑖2 ∙∝𝑖2 + 𝜐𝑖3 ∙∝𝑖3 + 𝜐𝑖4 ∙∝𝑖4 )

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(Eq. 14)

wcal =1- Vtotal

(Eq. 15)

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The model variables were obtained using the Solver function of Microsoft Excel by minimizing the following objective function (FO): 𝑒𝑥𝑝

𝑑𝑤𝑗

𝑑𝑡

−

𝑑𝑤𝑗𝑐𝑎𝑙 𝑑𝑡

)

(Eq. 16)

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𝐹𝑂 = ∑(𝑤𝑗𝑒𝑥𝑝 − 𝑤𝑗𝑐𝑎𝑙 ) + ∑ (

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3.1. Physic-chemical characterization of AS

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3. RESULTS AND DISCUSSIONS

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Table 1 shows the main characteristics of AS. It was observed that the moisture content was less than 3%. This value is interesting because the use of AS in combustion or

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pyrolysis processes will not require a previous step of drying. The content of volatiles

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and fixed carbon in the AS was similar to that of other types of biomass and they made the material suitable for its use as fuel (Montenegro, 2014; García et al., 2017). Ash is

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an important parameter which directly affects the heating value. High ash content makes biomass less appropriate as fuel. The low ash content of AS (0.65 %) led to high energy yields. Moreover, it prevents problems of slag formation which are caused by

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accumulation of ash residues in combustion camera and reduces cost of maintenance. Also, obtained values were lower than the ones obtained by Garcia et al., (2017), although they were in the range of the most of agricultural and forest wastes (Almendros et al., 2015).

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Table 1 also shows chemical composition of AS. Hot water-soluble fraction was 1.60%, meanwhile, the ethanol–benzene extractables content was around 0.34%. The contents of lignin and holocellulose were 49.81% and 48.25%, respectively. These results were in accordance with those found by others authors for different agro industrial wastes (Jauhiainen et al., 2004; Jiménez et al., 2007; Pirayesh and Khazaeian 2012). The high

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content of lignin and the low content of extractable compounds converted AS into a good biomass material for industrial application as fuel in combustion processes.

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Furthermore, some authors have indicated that pyrolysis of biomass with a high percentage of lignin can produce better bio-char yields (Yang et al., 2006).

With regard to elemental analysis of AS, it was mainly composed of carbon (44.80%)

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and oxygen (47.60 %). Carbon and hydrogen are thought to be the most important

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constituents of biomass. Almost all carbon and hydrogen contents in biomass are

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present in a combined form in complex organic compounds. Only a low percentage of

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nitrogen (0.43%) was detected and negligible value of sulfur (<0.10 %) was found.

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Sulfur content is an important parameter in the search for new fuels. From an environmental point of view, the emission of SO2 when burning combustibles is related

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with acid rain, which have an effect on crops and water bodies. In addition, sulfur dioxide can react to form sulfate particulates which cause asthma and bronchitis on

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vulnerable population. The results were similar to others lignocellulosic solids as pine cone shell or olive wastes, reported by other authors (Calero et al., 2013; Almendros et

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al., 2015; Volpe et al., 2015). Table 1 also shows the calorific values. Calorific values of the different fractions of the AS are high, which means that AS has a high potential as renewable fuel. It is worth noting that calorific value of the acid fraction (lignin) was a 22% higher than the native

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material. However, the oxidative fraction (holocellulose) yielded a lower value than the native AS. Finally, the results obtained for biosorption were previously reported by Martín-Lara et al., (2018b). The retained amount of metal by AS was different for each type of metal, varying between 4.5 mg/g for chromium and 20.4 mg/g for lead.

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3.2. Thermal decomposition in an inert atmosphere of nitrogen (pyrolysis)

Figure 1 shows the mass loss (w, defined as the mass fraction of solid, including the

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residue formed during the decomposition and the unreacted solid) versus temperature for each one of the isolated fractions from AS and the native AS in inert atmosphere of

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nitrogen and a heating rate of 15 K/min.

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The thermogravimetric curves (TG) and the derivative thermogravimetric curves (DTG)

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for AS pyrolysis showed three stages of mass loss. The first stage, which was around

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416-600K, corresponded to the degradation of the hemicellulose (Chen et al., 2015; Zabaniotou et al., 2010). Otherwise, the second stage, which appeared in the range of

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600-680K, corresponded to the degradation of the cellulose (Garima et al., 2015). Finally, the third stage, which was at temperatures higher than 1073 K, corresponded to

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degradation of lignin (Chen et al., 2012; Jeguirim et al., 2014).

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With respect to isolated fractions, fractions exhibited similar decomposition curves in a determined range of temperature. Only, acid fraction (lignin) revealed different degradation pathway. Also, a slight increase of oxidative fraction in degradation peaks

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compared with the native material was observed. In order to obtain the kinetic parameters of AS in a more optimized way, a calculation sequence was developed. Firstly, kinetic parameters of the simpler fractions were obtained and, then, these kinetic values were used in determination of kinetics of the most complex fractions. 14

Table 2 shows all the kinetic parameters of each fraction. It is important to highlight that secondary reactions between compounds as mineral salts, starch, tannins, resins, fats, gums, etc. were not taken into account in the kinetic model. Hence, small deviations were observed in kinetic parameters of native AS and its isolated fractions. Figures 1 (c) and 1 (d) show the fit of the proposed model for acid fraction. It was

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observed that the model fitted well the experimental data (R2 value of 0.9964). We could consider that this fraction was composed of a single solid which was lignin, since

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its kinetic behavior was very similar to the one described in the bibliography for lignin (Pérez et al., 2018). The small difference between 500 and 650 K could be due to the

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impurities in the material which were not removed during the isolation of lignin. The

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kinetic parameters of the acid fraction pyrolysis were similar to those investigated for

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lignin by other authors (Tang and Neil, 1964; Biagini and Tognotti, 2013).

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During pyrolysis of oxidative fraction (see Figures 1(e) and 1(f)), three independent compounds were degraded (two marked peaks and other lower at the end of the DTG

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curve were observed): hemicellulose, cellulose and lignin. It indicated that lignin was not completely attacked in the process of isolation and for that reason, a small amount

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was found in holocellulose fraction. Again, the proposed model fitted very well with the experimental data. The kinetic parameters of hemicellulose were similar to those

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investigated by other authors (Di Blasi, 2008; Biagini and Tognotti, 2013). As for the cellulose, the obtained kinetic parameters were very similar to those found by other

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authors who studied commercial cellulose and olive tree pruning cellulose (Antal et al., 2000; Pérez et al., 2018). The kinetic parameters obtained for lignin in the acid fraction were used and the pre-exponential constant and the activation energy were adjusted. Figures 1 (g) and (h) shows the TG and DTG curves of the fraction free of extracts. It was observed that the calculated TG curve almost perfectly matched the experimental

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TG curve with a correlation coefficient of 0.9996. Also, a composition of a 50% of lignin and other 50% of hemicellulose and cellulose was determined for this fraction. Other authors published similar composition for this solid (Antal et al., 2000), although the composition may vary widely depending on the origin of the AS. In the case of kinetic parameters of extract-free fraction, it was assumed that the

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activation energies of both hemicellulose and cellulose increased with respect to the oxidative fraction, while activation energy of the lignin decreased. Other authors

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performed kinetic studies for the AS. Nonetheless, its organic fractions were not taken

into account. The activation energies were in the range between 97.9 and 254.4 kJ/mol (Caballero et al., 1996) or 100 and 203.6 kJ/mol (Font el al., 1991).

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Finally, Figures 1(i) and 1(j) show experimental and calculated TG and DTG curves of

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native AS. Calculated TG curve adjusted properly the experimental TG curve with a

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correlation coefficient of 0.9993, as it shows Table 2. The kinetic parameters obtained

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for native AS were similar to those obtained by other authors (Font et al., 1991;

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Caballero et al., 1997; Orfão et al., 1998; Anca-Counce et al., 2014). Also, these authors presented similar yield coefficients of carbonaceous solid.

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Figure 1 (j) shows small deviations at maximums dw/dt values with hemicellulose and cellulose. The deviations could be due to the series of secondary reactions that have not

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been taken into account in the kinetic model which had been discussed previously (Anca-Couce et al., 2014). Furthermore, according to Marcilla et al. (2013), the amount

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of carbonaceous residue produced in each reaction depends on the particle size, as well as, the degree of compaction, two factors that could affect the fit of the kinetic model proposed.

16

Figure 3 shows the rate of degradation of the pyrolysis of the native AS and all fractions studied versus the temperature. In this figure α11 corresponds to the degree conversion of pyrolytic degradation of hemicellulose; α21 corresponds to cellulose and α31 to lignin. The first compound that was degraded was hemicellulose, followed by cellulose. Lignin was degraded during the whole temperature range. It was observed that the model

IP T

assigned a maximum rate of degradation to each component at a temperature that coincided with the maximum obtained experimentally in the DTG for each one of the

SC R

fractions studied. 3.3. Thermal decomposition in an air atmosphere (combustion)

Figure 2 shows that similar TG and DTG degradation peaks were found in combustion

U

and pyrolysis tests of AS. The procedure for the calculation of the kinetic parameters of

N

combustion followed the sequence of pyrolysis. Firstly, kinetic parameters of the easiest

M

parameters of complex fractions.

A

fractions were calculated. Later, those parameters were used to calculate kinetic

ED

Table 3 shows kinetic parameters of combustion calculated for describing the oxidative thermal decomposition of all isolated fractions of AS and native AS.

PT

Figure 2 (i) and 2(j) shows TG and DTG curves of native AS. An accurate fit of the calculated data to the experimental data was achieved, with the exception of the DTG

CC E

peak of the cellulose, where small differences were observed. Those differences may be due to the heterogeneity of AS or as aforementioned, because secondary reactions were

A

not taken into account. According to other author’s results, cellulose accelerates its decomposition in the presence of oxygen. For that reason, the peak of maximum degradation, corresponding to cellulose occurred at lower temperatures than in the pyrolysis process (Conesa and Domene 2011). Also, the combustion peak can shift when particle size is too small, which can produce the fusion of the peaks of

17

hemicellulose and cellulose (Marcilla et al., 2013). In this study, it was observed that combustion reactions did not occur for hemicellulose and for cellulose. It indicated that their decomposition occurred only by pyrolysis and then, volatile compounds were the ones which reacted with the oxygen present in the medium. Two well-differentiated degradation peaks appeared in the DTG for the acid fraction

IP T

(lignin). These peaks mostly corresponded to the combustion of lignin and to the

combustion of the char formed at high temperatures. This behavior was previously

SC R

described for lignin obtained from other lignocellulosic materials (Pérez et al., 2018; Ronda et al., 2017).

The oxidative fraction had a similar behavior to the native material, although the

U

degradation of cellulose in this case was more prominent because the cellulose content

N

was higher than for the native sample. The data adjustment was very food and

A

reproduced correctly the degradation profiles of the three components. It predicted the

M

behavior of TG and DTG curves of the fraction free of extracts with very small errors.

ED

For this fraction, the degradation behavior was similar to that observed for the oxidative fraction. In fact, only a change in the hemicellulose and cellulose degradation maxima

PT

was observed at high temperatures.

Figure 3 shows degradation rates of native AS and all the fractions studied, where α11

CC E

corresponds to degradation of pyrolysis reaction of hemicellulose; α21 corresponds to degradation of pyrolysis reaction of cellulose; α31 corresponds to lignin pyrolysis

A

reaction; α32 corresponds to degradation of combustion reaction of lignin; α33 corresponds to degradation of combustion reaction of carbonaceous residue produced in lignin pyrolysis reaction and α34 corresponds to degradation of combustion reaction of carbonaceous residue from lignin combustion reaction. It was observed that the model assigned a maximum rate of degradation to each component at a temperature that

18

coincided with the maximum obtained experimentally in the DTG for each one of the fractions studied. 3.4. Effect of metals on thermal decomposition of AS Figures 4 and 5 show TG and DTG curves of metal-loaded AS samples. All the impregnated samples degraded at higher temperatures than the native material. Tables 4

IP T

and 5 show kinetic parameters of pyrolysis and correlation coefficients of metal-loaded AS. It was observed for all studied cases that the kinetic parameters found were very

SC R

similar to kinetic parameters of extracts-free fraction sample, and therefore, the reactions were slower with respect to the native AS. This behaviour could be due to the

previous stage of biosorption of metal was performed on aqueous solution, which led to

U

the dissolution of water soluble compounds.

A

accordance with native AS in combustion.

N

On the other hand, the kinetic degradation of lignin (Figure 5) fraction was more in

M

The only parameter that changed almost insignificantly was the activation energy that

ED

increased for all fractions, but just a small variation. Other authors also found similar increases in activation energies when a metal-loaded sample was analyzed (Helsen and

PT

Van den Bulck, 2000; Lievens et al., 2008; Martín-Lara et al., 2016). Finally, slight shifts of the hemicellulose and cellulose peaks were observed in this work and in other

CC E

previous investigations (Guicai et al., 2016). However, this behaviour was insignificant and for that reason, it could be concluded that the studied metals did not seem to affect

A

thermal decomposition of AS.

4. CONCLUSIONS

19

The kinetic parameters of the pyrolysis of the AS were similar to the ones obtained with other lignocellulosic biomass compounds (preexponential, activation energy, reaction order and proportion of carbonaceous residue formed), being in the ranges reported by other authors. The conversion in inert nitrogen atmosphere and oxygen atmosphere process were

IP T

similar for temperatures around 633 K. In the combustion processes with 20% of oxygen, combustion behaviour shifted to higher temperatures and these oxidative

SC R

processes were characterized by their low rates of devolatilization.

With regard to the effect of the presence of metals (cadmium, copper, chromium, nickel and lead) in the AS, it could be concluded that the kinetics of these metals did not affect

U

the thermal decomposition, since the kinetic parameters were very similar to the free of

N

extracts fraction sample. This could be due to the previous stage of adsorption carried

A

out for the impregnation of metals. This step was performed in an aqueous medium,

A

CC E

PT

ED

M

which resulted in the loss of extractable compounds from the material.

20

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IP T

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PT

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CC E

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27

Figure 1: TG and DTG curves of AS and its isolated fractions in an inert atmosphere of nitrogen (pyrolysis). (c) and d) acid fraction; e) and f) oxidative fraction, g) and h) extractives-free AS; i) and j) native AS)

1.0

0.0025

AS Native AS extracts free Oxidative fraction Acid fraction

0.9 0.8

AS Native AS extracts free Oxidative fraction Acid fraction

0.0020

dw/dt, s-1

0.7

w

0.6 0.5 0.4

0.0015

0.0010

0.3 0.2

0.0005

0.0 423

473

523

573

623

a)

673

723

773

823

873

923

IP T

0.1

0.0000

973 1023

423

Temperature, K

473

523

573

623

b)

673

723

773

823

873

Temperature, K

1.0

Experimental model Calculated model

0.8

0.0020

dw/dt, s-1

0.7

w

0.6 0.5 0.4

0.0015

0.0010

0.3 0.2

U

0.0005

0.1 0.0 473

523

573

623

673

723

773

823

873

923

0.0000

973 1023

423

Temperature, K

A

e) Experimental model Calculated model

0.8 0.7

0.5 0.4 0.3 0.2 0.1 0.0 423

473

523

573

623

673

723

773

823

523

573

873

923

PT

723

773

823

873

923

973 1023

Experimental model Calculated model

0.0025

0.0015

0.0010

0.0005

0.0000 423

973 1023

473

523

573

623

673

723

773

823

873

923

973 1023

Temperature, K

0.0025

Experimental model Calculated model

CC E

0.9

673

Temperature, K

f)

Temperature, K

1.0

623

0.0020

ED

w

0.6

M

1.0 0.9

473

N

d)

dw/dt, s-1

423

c)

973 1023

SC R

0.0025

Experimental model Calculated model

0.9

923

Experimental model Calculated model

0.8

0.0020

dw/dt, s-1

0.7

w

0.6 0.5 0.4

0.0015

0.0010

0.3 0.2

0.0005

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0.1 0.0

423

g)

473

523

573

623

673

723

773

823

873

923

0.0000

973 1023

423

Temperature, K

h)

28

473

523

573

623

673

723

773

823

Temperature, K

873

923

973 1023

1.0

Experimental model Calculated model

0.9

Experimental model Calculated model

0.0025

0.8 0.0020

dw/dt, s-1

0.7

w

0.6 0.5 0.4

0.0015

0.0010

0.3 0.2

0.0005

0.1 0.0000

0.0 423

523

573

623

673

723

773

823

873

923

423

973 1023

473

523

573

623

j)

Temperature, K

673

723

773

823

873

923

973 1023

Temperature, K

A

CC E

PT

ED

M

A

N

U

SC R

IP T

i)

473

29

Figure 2: TG and DTG curves of AS and its isolated fractions in an atmosphere of air (combustion). (c) and d) acid fraction; e) and f) oxidative fraction, g) and h) extractives-free AS; i) and j) native AS)

1.0

AS Native AS extracts free Oxidative fraction Acid fraction

0.9 0.8

0.0025

0.0020

dw/dt, s-1

0.7 0.6

w

AS Native AS extracts free Oxidative fraction Acid fraction

0.5 0.4

0.0015

0.0010

0.3 0.2

IP T

0.0005

0.1 0.0000

0.0 423

473

523

573

623

a)

673

723

773

823

873

923

423

973 1023

473

dw/dt, s-1

w

0.4

723

773

823

873

923

973 1023

SC R

0.0020 0.7

0.5

673

Experimental model Calculated model

0.0025

Experimental model Calculated model

0.8

0.6

623

Temperature, K

1.0

0.0015

0.0010

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0.3 0.2

0.0005

0.1 0.0

0.0000 473

523

573

623

c)

673

723

773

823

873

923

973 1023

423

Experimental model Calculated model

M

A

1.0

0.7 0.6 0.5

0.1 0.0 423

473

e)

523

573

623

673

ED

0.4

0.2

723

773

823

873

923

PT

673

723

773

823

873

923

973 1023

Temperature, K

0.0025

Experimental model Calculated model

0.0015

0.0010

0.0000 423

973 1023

473

523

573

f)

623

673

723

773

823

873

923

973 1023

Temperature, K

0.0025

Experimental model Calculated model

Experimental model Calculated model

CC E

0.9

623

0.0005

Temperature, K

1.0

573

0.0020

dw/dt, s-1

0.8

0.3

523

d)

Temperature, K

0.9

473

N

423

w

573

b)

Temperature, K

0.9

523

0.8

0.0020

dw/dt s-1

0.7

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0.6 0.5 0.4

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0.0010

0.3

0.0005

0.2

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0.1

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0.0 423

g)

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623

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723

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823

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h)

Temperature, K

30

473

523

573

623

673

723

773

823

Temperature, K

873

923

973 1023

1.0

Experimental model Calculated model

0.9

0.0025

Experimental model Calculated model

0.8 0.0020

dw/dt, s-1

0.7

w

0.6 0.5 0.4

0.0015

0.0010

0.3 0.2

0.0005

0.1 0.0000

0.0 423

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623

i)

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723

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823

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923

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973 1023

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823

873

923

973 1023

Temperature, K

A

CC E

PT

ED

M

A

N

U

SC R

IP T

Temperature, K

31

Figure 3: Development of proposed reactions in function of temperature for AS and its isolated fractions. (a), c), e), g), and i) pyrolysis; b), d), f), h) and j) combustion)

0.007

0.007



0.006

0.006

0.004

0.003

0.003

0.002

0.002

0.001

0.001

0.000

 

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a)

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b)

Temperature, K

 

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 

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0.004

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 

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Temperature, K

0.007

0.006

623

SC R

423

d/dt, s-1

IP T

d/dt, s-1

d/dt, s-1

 

0.005

0.005

0.003

0.002

N

0.002 0.001

0.001

0.000

0.000

523

573

623

c)

673

723

773

823

873

923

973 1023

Temperature, K



0.003

0.002

0.000 423

473

e)

PT

0.001

523

573

623

673

723

773

823

523

573

623

673

723

773

823

873

923

973 1023

Temperature, K

0.007

 

0.006

 

0.005

ED

0.005

M

 

0.006

0.004

473

d)

0.007

d/dt, s-1

423

A

473

d/dt, s-1

423



0.004

0.003

0.002

0.001

0.000 873

923

973 1023

423

Temperature, K

473

523

573

CC E

f)

623

673

723

773

823

873

923

973 1023

Temperature, K

0.007

0.007

 

0.006

d/dt, s-1

d/dt, s-1

A

 



0.005

0.004

0.003

0.002

0.005



0.004 0.003 0.002

0.001

0.001

0.000 423

g)

 

0.006

473

523

573

623

673

723

773

823

873

923

0.000

973 1023

423

Temperature, K

h)

32

473

523

573

623

673

723

773

823

Temperature, K

873

923

973 1023

Figure 4: Comparative TG and DTG curves of pyrolysis of native AS and extracts-free AS and metal-loaded AS.

1.0

0.0020

0.9

AS Native AS-Cd AS-Cu AS-Cr AS-Ni AS-Pb

0.7

w

0.6

AS Native AS-Cd AS-Cu AS-Cr AS-Ni AS-Pb

0.0015

dw/dt s -1

0.8

0.5 0.4

0.0010

0.0005

0.2 0.1 0.0000

0.0 423

473

523

573

623

673

723

773

823

873

923

423

973 1023

473

523

573

623

0.0020

AS extracts free AS-Cd AS-Cu AS-Cr AS-Ni AS-Pb

w

0.6

773

823

0.4

973 1023

0.0010

N

0.3

923

AS extracts free AS-Cd AS-Cu AS-Cr AS-Ni AS-Pb

0.0015

0.5

873

U

0.7

dw/dt s -1

0.8

723

SC R

1.0 0.9

673

Temperature, K

Temperature, K

IP T

0.3

0.0005

0.2

A

0.1

0.0000

0.0 423

473

523

573

623

673

723

773

823

873

923

973 1023

A

CC E

PT

ED

M

Temperature, K

33

423

473

523

573

623

673

723

773

823

Temperature, K

873

923

973 1023

Figure 5: Comparative TG and DTG curves of combustion of native AS and extractsfree AS and metal-loaded AS.

1.0

AS Native AS-Cd AS-Cu AS-Cr AS-Ni AS-Pb

0.8 0.7

w

0.6

0.0020

AS Native AS-Cd AS-Cu AS-Cr AS-Ni AS-Pb

0.0015

dw/dt s -1

0.9

0.5 0.4

0.0010

0.0005

0.2 0.1 0.0000

0.0 423

473

523

573

623

673

723

773

823

873

923

423

973 1023

473

523

573

623

AS extracts free AS-Cd AS-Cu AS-Cr AS-Ni AS-Pb

w

0.6

823

0.5 0.4

923

973 1023

0.0015

0.0010

N

0.3

873

U

0.7

773

AS extracts free AS-Cd AS-Cu AS-Cr AS-Ni AS-Pb

0.0020

dw/dt s -1

0.8

723

SC R

1.0 0.9

673

Temperature, K

Temperature, K

IP T

0.3

0.0005

0.2

A

0.1

0.0000

0.0 423

473

523

573

623

673

723

773

823

873

923

973 1023

A

CC E

PT

ED

M

Temperature, K

34

423

473

523

573

623

673

723

773

823

Temperature, K

873

923

973 1023

Table 1: Characterization of native AS and isolated fractions of AS Analysis

analysis

Chemical

Moisture (%)

Volatile matter (%)

Fixed carbon (%)

Ashes (%)

2.96

71.90

24.50

0.65

Hot water soluble

Ethanol soluble

Extract-free

compounds* (%)

compounds* (%)

lignin*(%)

8.18

1.60

0.34

50.80

%C

%H

%N

%S

44.80

7.10

0.43

Native AS

Extractive-free AS

4,015

4,314

Moisture (%)

Calorific value (kcal/kg)

Oxidative fraction of AS

U

analysis

3,996

N

Elemental

SC R

analysis

A

CC E

PT

ED

M

A

*Dry basis

35

< 0.1

Acid fraction of AS

4,916

Extract-free holocellulose*

IP T

Proximate

Characteristic parameters of the almond shell

(%)

49.20 %O

47.60

Table 2: Kinetic parameters of pyrolysis of AS and its isolated fractions.

Kinetic parameter E, kJ/mol n

k0, s-1

c

Acid fraction from almond shell R2 =0.9964, R2 derivate= 0.9113 Lignin (α31)

2.73E+04

85.36

4.00

0.48

Hemicellulose (α11) Cellulose (α21) Lignin (α31)

1.55E+09 4.44E+14 1.01E+04

117.8 192.1 87.59

1.02 0.96 4.00

0.00 0.21 0.48

Extractive-free almond shell R2 =0.9996, R2 derivate= 0.9962 1.55E+09 4.44E+14 2.79E+04

123.7 202.2 81.58

1.02 0.99 4.00

0.00 0.14 0.48

SC R

Hemicellulose (α11) Cellulose (α21) Lignin (α31)

Native almond shell R2 =0.9993, R2 derivate= 0.9898 121.9 197.9 81.74

A M ED PT CC E A

36

1.02 1.00 4.00

U

1.55E+09 4.44E+14 2.75E+04

N

Hemicellulose (α11) Cellulose (α21) Lignin (α31)

IP T

Oxidative fraction from almond shell R2 =0.9995, R2 derivate= 0.9870

0.00 0.14 0.48

Table 3: Kinetic parameters of combustion of AS and its isolated fractions.

k0, s-1 E, kJ/mol n c 2 2 1. Acid fraction combustion R =0.9999, R derivate= 0.9916 α31 2.73E+04 85.36 4.00 0.48 α32 2.84E+04 37.02 1.42 0.61 Lignin α33 2.68E+06 88.51 1.23 0.00 α34 7.49E+03 42.53 1.23 0.00 2 2 2. Oxidative fraction combustion R =0.9984; R derivate=0.9831 α 11 1.55E+09 116.4 1.02 0.00 Hemicellulose α 21 4.44E+14 186.7 0.96 0.21 Celulose α31 2.59E+04 85.42 2.97 0.50 α32 2.83E+04 37.02 0.61 0.80 Lignin α34 7.43E+03 42.56 3.96 0.00 2 2 3. Extractive free simple combustion R =0.9997; R derivate=0.9961 α 11 1.55E+09 121.6 1.02 0.00 Hemicellulose α 21 4.44E+14 193.9 0.99 0.14 Celulose α31 2.61E+04 84.50 2.96 0.50 α32 2.86E+04 35.74 0.60 0.80 Lignin α34 7.62E+03 42.36 3.96 0.00 2 2 4. Almond shell combustion R =0.9999; R derivate=0.9772 α 11 1.55E+09 119.6 1.02 0.00 Hemicelullose α 21 4.44E+14 190.9 0.99 0.14 Celulose α31 2.55E+04 87.07 4.81 0.31 α32 2.86E+04 35.33 1.12 0.22 Lignin α34 9.59E+03 39.27 2.06 0.00

b 5.73 5.99 5.61 7.27 0.10

A

CC E

PT

ED

M

A

N

U

SC R

IP T

Parameter

37

7.27 0.10 6.70 5.39

Table 4: Kinetic parameters of pyrolysis of metal-loaded AS. k0, s-1

E, kJ/mol

n

c

Cadmium R2=0.9996; R2derivate=0.9912 Hemicellulose (α11)

1.55E+09

123.3

1.02

0.00

Cellulose (α21)

4.44E+14

202.0

0.99

0.14

Lignin (α31)

2.73E+04

84.46

4.00

0.48

2

2

Hemicellulose (α11)

1.55E+09

123.8

1.02

0.00

Cellulose (α21)

4.44E+14

203.1

0.99

0.14

Lignin (α31)

2.78E+04

81.81

4.00

0.48

2

2

Chromium R =0.9997; R derivate=0.9962 1.55E+09

124.2

1.02

0.00

Cellulose (α21)

4.44E+14

203.1

0.99

0.14

Lignin (α31)

2.78E+04

82.08

4.00

2

SC R

Hemicellulose (α11)

2

0.48

Nickel R =0.9991; R derivate=0.9942 1.55E+09

123.9

1.02

0.00

Cellulose (α21)

4.44E+14

203.5

0.99

0.14

81.15

4.00

0.48

2.81E+04 2

2

U

Hemicellulose (α11) Lignin (α31)

Lead R =0.9996; R derivate=0.9923 Cellulose (α21)

4.44E+14

Lignin (α31)

2.70E+04

123.2

N

1.55E+09

201.5

A

CC E

PT

ED

M

A

Hemicellulose (α11)

38

85.92

IP T

Copper R =0.9997; R derivate=0.9949

1.02

0.00

0.99

0.14

4.00

0.48

Table 5: Kinetic parameters of combustion of metal-loaded AS.

k0, s-1

E, kJ/mol 2

n

c

b

2

Cadmium R =0.9999; R d=0.9964 α 11

1.55E+09

121.7

1.02

0.00

-

Celulose

α 21

4.44E+14

195.5

0.99

0.14

-

α31

2.51E+04

89.12

4.81

0.31

-

α32

2.80E+04

38.89

1.12

0.22

6.70

9.39E+03 39.61 2 2 Copper R =0.9999; R d=0.9865

2.06

Lignin

0.00

5.39

0.00

-

0.14

-

α 11

1.55E+09

121.2

1.02

Celulose

α 21

4.44E+14

194.2

0.99

α31

2.50E+04

89.36

4.81

0.31

-

α32

2.80E+04

39.09

1.12

0.22

6.70

2.06

0.00

5.39

122.2

1.02

0.00

-

194.5

0.99

0.14

-

Lignin

α34

SC R

Hemicelullose

U

α34

IP T

Hemicelullose

9.36E+03 39.65 Chromium R2=0.9997; R2d=0.9888

α 11

1.55E+09

Celulose

α 21

4.44E+14

α31

2.55E+04

87.54

4.81

0.31

-

α32

2.83E+04

37.48

1.12

0.22

6.70

9.46E+03

39.45

2.06

0.00

5.39

α34

A

M

Lignin

N

Hemicelullose

α 11

1.55E+09

121.8

1.02

0.00

-

Celulose

α 21

4.44E+14

195.0

0.99

0.14

-

α31

2.55E+04

87.09

4.81

0.31

-

α32

2.81E+04

38.07

1.12

0.22

6.70

α34

9.29E+03

39.70

2.06

0.00

5.39

CC E

Lignin

PT

Hemicelullose

ED

Nickel R2=0.9999; R2d=0.9938

2

2

Lead R =0.9997; R d=0.9917

Hemicelullose

α 11

1.55E+09

121.7

1.02

0.00

-

Celulose

α 21

4.44E+14

195.4

0.99

0.14

-

α31

2.52E+04

87.70

4.81

0.31

-

α32

2.83E+04

37.83

1.12

0.22

6.70

α34

9.66E+03

39.15

2.06

0.00

5.39

A

Lignin

39