Waste Management xxx (2015) xxx–xxx

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Thermal degradation of paper industry wastes from a recovered paper mill using TGA. Characterization and gasification test Jorge Arenales Rivera a,⇑, Virginia Pérez López a, Raquel Ramos Casado a, José-María Sánchez Hervás b a b

Department of Energy, CEDER-CIEMAT, Autovía de Navarra A15, Salida 56, Soria 42290, Spain Department of Energy, CIEMAT, Avda. Complutense 40, Madrid 28040, Spain

a r t i c l e

i n f o

Article history: Available online xxxx Keywords: Characterization Circulating fluidized bed Paper industry waste Pelletizing Refuse derived fuel Thermal degradation

a b s t r a c t In this survey, a refuse derived fuel (RDF) was produced from paper industry wastes through a mechanical treatment (MT). The two main wastes generated from a recovered paper mill were rejects and de-inking sludge, which were produced principally in the pulping and de-inking processes, respectively. This work presents raw wastes characterization, fuel preparation and gasification tests performed in a circulating fluidized bed (CFB) gasifier pilot plant. The characterization was carried out by proximate and ultimate analysis. Several blends of pre-conditioned rejects and de-inking sludge were densified by means of pelletizing, studying the energy consumption and its quality properties. Besides, thermal degradation of blends was studied under thermogravimetric analysis (TGA). The experimental runs were made from 30 to 900 °C in nitrogen atmosphere at three heating ranges, b = 5, 10 and 20 °C/min. Two thermal stages were identified during the thermal degradation, which are linked to cellulose and plastic degradation. In addition, kinetics parameters were estimated by the application of non-isothermal methods: Kissinger–Akahira–Sunose (KAS), Flynn–Ozawa–Wall (FOW) and Coats and Redfern. The activation energy values were about 140–160 kJ/mol and 60–80 kJ/mol for plastic and cellulosic materials, respectively. Regarding waste valorisation, a blend composed of 95% of rejects and 5% of de-inking sludge was selected for gasification tests. The energy consumption during the preparation was recorded and a gasification tests were done to prove the usability of these pellets in a CFB gasifier. The main results were a net calorific value (NCV) of 5 MJ/Nm3 and a total tar content of 11.44 g/Nm3 at an equivalence ratio (ER) of 0.3. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction In advance societies, the continuous generation of wastes has become such a big problem that reducing the amount of them either by reusing or recycling at any levels is necessary, as it has been shown in different programmes such as the National Integrated Waste Plan 2008–2015 in Spain (MAGRAMA, 2013) or through European laws as Directive 2008/98/EC. The purpose of these programmes and laws is to convert the European Union in a ‘‘recycling society’’ (Arena, 2015; Martínez et al., 2013; Rada et al., 2014), which can reduce the waste generation and use them as a source for different processes (Ionescu et al., 2013; Martínez et al., 2012; Premakumara et al., 2011; Singh et al., 2014). In this context, new waste management strategies as increasing selective ⇑ Corresponding author. Tel.: +34 975281013. E-mail addresses: [email protected], [email protected] (J. Arenales Rivera), [email protected] (V. Pérez López), raquel.ramos@ ciemat.es (R. Ramos Casado), [email protected] (J.-M. Sánchez Hervás).

collection (SC), e.g. implementing vacuum waste collection systems (Ciudin et al., 2014), may improve waste management issues (Petrescu et al., 2010). Nowadays, paper mill industries produce lots of wastes, which are removed by landfilling or incinerating. In 2010, total paper production in Europe was 96.5 million tonnes which generated 8.9 million tonnes of waste (CEPI, 2010) which decreased compared to 11 million tonnes produced in 2005 (Monte et al., 2009). Also, the production of recycled paper, during the same period, was 49 million tonnes generating 8.2 million tonnes of solid waste (CEPI, 2010). In addition, not only these companies have to deal with the wastes that are produced in the process, but also with the huge amount of wastes that are generated by classification and preparation of feedstock. Paper mills that work with recovered feedstock reject lots of wastes which contain components with high calorific value such as plastic materials, cardboard and paper waste. Nasrullah et al., studied the production of secondary fuels from commercial and industrial waste which presented a plastic material content about 29% and paper and cardboard content of

http://dx.doi.org/10.1016/j.wasman.2015.04.031 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Arenales Rivera, J., et al. Thermal degradation of paper industry wastes from a recovered paper mill using TGA. Characterization and gasification test. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.04.031

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J. Arenales Rivera et al. / Waste Management xxx (2015) xxx–xxx

31% (Nasrullah et al., 2014). Also, others wastes such as de-inking sludge or sludge from wastewater treatment are produced from de-inking process and effluents treatment. The paper mill industries that use recovered paper to produce recycled paper, generate mainly de-inking sludge, about 90% of total sludge, and a small amount of rejects compared with the sludge production. Before the economic crisis, the sludge from paper mill industries was used in significant quantities as filler for the cement industry (Ahmadi and Al-Khaja, 2001; Frías et al., 2015; Yan et al., 2011). However, the current practice for the disposal of sludge is landfilling. Nevertheless, these wastes could be converted into energy by means of thermal processes instead of being landfilled. This would be in accordance with the recycling and recovery targets to be accomplished by 2020: 50% preparing for re-use and recycling of certain waste materials from households and other origins similar to households. Therefore, the production of a normalized combustible derived fuel from wastes in Europe is of great interest in waste management (Rada and Andreottola, 2012; Rotter et al., 2011), not only for an economic, but also for an environmental point of view (Wu et al., 2002). Currently, Spain is the sixth leading paper producing industry in Europe. This meant a yielding around 7.4 million tonnes in 2010 (Ruiz Peñalver et al., 2014), which produced a high amount of wastes. The amount of paper industry wastes generated in 2010 was about 1.7 million tonnes (ASPAPEL, 2014), despite this Spanish paper industry is the second larger paper recycling industry in Europe only below Germany. Hence, it is necessary to set up different strategies for waste management to reduce the amount of waste sent to disposal. In recent years, there has been a growing interest in develop new technologies for the production of combined heat and power (CHP), being one of the most promising waste gasification process. The reduction of fossil CO2 emissions could be feasible, so that biodegradable material content of wastes can be considered as a renewable energy source according to the Directive 2009/28/EC. Therefore, recovering useful thermal energy from these wastes can be hold as an acceptable solution. Gasification is the thermal conversion of a solid fuel into a gas with heat and chemical energy, which is composed of combustible gases such as CO, H2 and CH4. Although there are lots of studies about application of waste gasification in fluidized bed gasifiers, few of them are related to paper industry wastes, being mainly about paper sludge (Liu et al., 2012; van der Drift et al., 2001; Xie and Ma, 2012) instead of blends of rejects and de-inking sludge. Nevertheless, other studies, that showed good results, were done on blends of rejects and de-inking sludge from paper mills with wood chips in a downdraft gasifier (Ouadi et al., 2013). The main objective pursued in the present study, was to find a suitable route to recover energy from paper industry wastes in a fluidized bed gasifier from blends of rejects and de-inking sludge. Hence, it presents details about the experimental data obtained and the assessment of the possibility of wastes energy conversion from a recovered paper mill industry. The paper shows the main stages to obtain a RDF. Firstly, raw wastes characterization, which are carried out by TGA, proximate and ultimate analysis. Secondly, pelletizing process, where four blends were produced by adding de-inking sludge from 5% to 35%. In this stage, the physical properties were determined to ensure the pellet quality (Sarc and Lorber, 2013) as well as the fuel characterization of each blend. Besides, the kinetics parameters were estimated to understand the behavior of fuels when they are under combustion or gasification processes which occur at high temperatures. There are two types of experimental setups in order to obtain TG data for estimating kinetic parameters, non-isothermal and isothermal processes. In this study, non-isothermal methods were used to obtain the Arrhenius

parameters. Finally, one of the blends was used as fuel in a CFB gasifier pilot plant in order to explore the feasibility of the process, recording the main gasification parameters as well as tar and gas compositions. 2. Material and methods 2.1. Raw wastes Paper industry wastes were received from Holmen Paper, an industry located in Fuenlabrada area, which is close to Madrid. Two different types of wastes were generated from the paper mill: rejects and de-inking sludge. Blends of these wastes were pelletized to produce fuels for thermal conversion using fluidized bed technologies. 2.2. Rejects Rejects is the term used to refer the waste fractions that are rejected from sorting, i.e. classification of the recovered paper from SC (Rada and Ragazzi, 2014) used as feedstock in the paper mill and also, from the initial screening before the pulper. Rejects from recovered paper mills are quite heterogeneous and variable, and consist mainly of lumps of fibres, staples from ring binders, sand, glass and plastics. The composition of rejects depends on the paper manufacturer’s process. However, rejects have high moisture content, being easily dewatered, significant calorific value because of plastic content and some impurities to remove such as: sand, glass or metals. 2.3. De-inking sludge De-inking sludge refers to the waste fraction generated during the de-inking process. De-inking sludge contains mainly short fibres, coatings, fillers such as kaolin (Al2O3, SiO2, H2O), talc (Mg3Si4O10 (OH)2),calcium carbonate (CaCO3) and clays that are added to improve finished properties of the paper product, additionally there are ink particles, extractive substances and de-inking additives. De-inking sludge has also a high moisture content that is cut down to 30–40wt.%, a low calorific value (4–7 MJ/kg) and a high ash content that is approximately 60–70wt.%, which is mainly calcium based. Besides, two more sludge streams are generated during the paper manufacturer’s process, corresponding to primary sludge, which contains heavy, fibrous and inorganic solids removed in the primary clarifier, and secondary sludge, referring to the wastewater treatment (Monte et al., 2009). 2.4. Characterization The samples were prepared in the laboratory by means of homogenization, grinding and drying. The standard methods used for the characterization are presented in Table 1. 2.5. Densification Blends of rejects and de-inking sludge were densified after a pre-conditioning treatment, which consists in ferrous/non-ferrous metal separation and stones/sand removal. The pelletizing process was optimized to ensure acceptable quality properties of fuels. The quality properties of pellets was evaluated in terms of moisture content, bulk density (CEN/TS 15401) and durability (CEN/TS 15639). 2.5.1. Comminution equipment The comminution equipment used was: a primary grinder, that was a double axis shredder with 22 kW drive power, and a

Please cite this article in press as: Arenales Rivera, J., et al. Thermal degradation of paper industry wastes from a recovered paper mill using TGA. Characterization and gasification test. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.04.031

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J. Arenales Rivera et al. / Waste Management xxx (2015) xxx–xxx Table 1 Standard methods used for laboratory characterization. Parameter

Standard methods

Moisture

EN 15414-3

Proximate analysis Volatile matter Ash Volatile matter Ash

EN EN EN EN

Ultimate analysis C, H, N S, Cl Major elements Ash melting behavior Calorific value

EN 15407 EN 15408 EN 15410 CEN/TR 15404 EN 15400

15402 15403 15402 15403

secondary grinder, which was a double bearing horizontal axis knife mill with a drive of 35 kW power and 640 r.p.m. rotation speed. The exchangeable insert screen had an opening in the upper part to feed the material into the chamber perpendicularly to the rotor axis. The rotor carried nine knives and six counter-knives that reached a peripheral velocity of 20 m/s. The inner diameter of the grinding chamber was 600 mm. 2.5.2. Pellet plant The facility used in the experiments was a pellet plant with capacity of 300 kg/h. The pellet press had a flat type Amandus Kahl 33-500: die diameter 500 mm, roller width 75 mm, drive power 30 kW. Besides, the press was equipped with a hydraulic pressure system able to generate a pressure up to 110 bar. The pellet plant has also a blending system, a pellet mill and cooling and bagging equipment. 2.6. Thermal degradation experiments TGA was done in an inert atmosphere of nitrogen simulating a pyrolysis process, due to the high volatile matter content of these materials. Weight loss was studied under different temperatures by TGA and simultaneously differential thermal analysis (SDTA). Mettler TGA/SDTA 851e (Mettler Toledo Corporation, Switzerland) TG analyzer was used to measure and record mass and temperature changes versus temperature over a simulated pyrolysis process. The characteristics of the analyzer are: Sensitive microbalance. 1 lg resolution. 1300 °C maximum temperature at atmospheric pressure. The system was connected to a personal computer for data recording and analysis. TGA was done at three different heating rates (5, 10 and 20 °C/min) from 30 to 900 °C. Nitrogen was used as purge gas to keep an inert atmosphere based on nitrogen in the pyrolysis zone (Li et al., 2005), cutting down unwanted oxidation conditions of the sample. Samples were placed in a platinum pan avoiding the contact with both sides of the oven. Previously to TGA, temperature, weight and platform calibrations were carried out. Trials were running at least twice in order to ensure the reproducibility of experimental data. Nevertheless, samples were air dried and blended in order to achieve homogeneity, i.e. samples of around 40 mg with particles sizes ranging between 0.1 and 0.2 mm were placed in the analyzer for each experimental run. 2.7. Kinetic study Knowledge of pyrolysis becomes important due to it is a key conversion during gasification or combustion of fuels such as

biomass and wastes. Pyrolysis is a previous stage for others thermochemical processes as mentioned above where a solid particle undergoes a thermochemical decomposition to produce a range of useful products. During pyrolysis, large complex hydrocarbon molecules break down into smaller and simpler molecules of gas, liquid and char. Nevertheless, pyrolysis is a complex process. TGA is a thermoanalytical technique that is widely used in solid-phase thermal decomposition studies (Mishra and Bhaskar, 2014), though a better understanding of the pyrolysis reaction sequence and mechanisms may be achieved by the combination of gas component detection methods with thermal analysis (Ciuta et al., 2014). The accuracy of kinetic parameters estimated from kinetic analysis mainly depend on the evaluation methods used to study the decomposition behavior of waste under different conditions of temperature and atmosphere. In this case, kinetic study of weight loss of wastes was done under nitrogen atmosphere, simulating a pyrolysis process. The fundamental kinetic equation of heterogeneous solid-state thermal transformation at a constant temperature heating rate (Ounas et al., 2011) may be described as follows:

da ¼ KðTÞ  f ðaÞ dt

ð1Þ

where K(T) is a temperature-dependent reaction rate constant, expressed by Arrhenius equation, and f (a) is a function that depends on the extent of conversion. The experimental study may be carried out by isothermal and non-isothermal methods. The main advantages of non-isothermal methods regarding to isothermal methods are the possibility to obtain results in a wide range of temperatures studying the influence of different heating rates on the thermal decomposition process. However, there are discrepancies on determination of kinetics coefficients by dynamic experiments because of kinetic compensation effect, thus it is obligated the study of heating rate influence (Ceamanos et al., 2002). For non-isothermal conditions, when temperature varies at a constant heating rate, Eq. (1) can be described as follows:

  da Ea  f ðaÞ ¼ A  exp  dT RT

b

ð2Þ

where A is the pre-exponential factor, Ea the activation energy, T the absolute temperature, R the gas constant and b is the heating rate, which is defined as b = dT/dt. Hence, Eq. (2) can be used for describing the time evolution of a physical or chemical change. Besides, f(a) can be describe as:

f ðaÞ ¼ ð1  aÞn

ð3Þ

where n is the order of reaction. Reordering Eq. (2) and Eq. (3) and upon integration, the reaction rate can be described as:

Z

x

x0

dx A ¼  ð1  aÞn b

Z

T

T0

exp 

  Ea  dT RT

ð4Þ

The first part of Eq. (4) is the integral form of the reaction model and the second part, which does not have analytical solution, correspond with the temperature integral. Therefore, the estimation of the value of integral temperature is based on the application of different methods that allow to obtain the Arrhenius parameters. Generally, non-isothermal methods are divided into two types: model-adjust and model-free methods, commonly called isoconversional methods (Slopiecka et al., 2012). In regard to model-fitting methods, the selection of the method that gives the best statistical fit is based on experimental data. On the other hand, model-free methods require TG curves at different heating rates so

Please cite this article in press as: Arenales Rivera, J., et al. Thermal degradation of paper industry wastes from a recovered paper mill using TGA. Characterization and gasification test. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.04.031

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J. Arenales Rivera et al. / Waste Management xxx (2015) xxx–xxx

that several kinetics curves are obtained allowing calculate the activation energy for each conversion extent. 2.7.1. Model-adjust methods Coats and Redfern method is a model-fitting integral method that eliminates the reaction rate constant and allow obtaining the Arrhenius parameters directly (Singh Chouhan et al., 2013). Using this method it is possible to estimate a value of the activation energy for each heating rate used in the experimental runs.

1  ð1  aÞ

ln

ð1nÞ

!



   AR 2RT Ea  1  bEa Ea RT

¼ ln

T 2 ð1  nÞ

ð5Þ

A simplification is assumed for Eq. (5) and gives Eq. (6):

1  ð1  aÞð1nÞ

ln

T 2 ð1  nÞ

! ¼ ln



 AR Ea  bEa RT

ð6Þ

2.7.2. Model-free methods FOW method is an isoconversional integral method. This method uses the Doyle’s approximation (Aboulkas et al., 2009), obtaining Eq. (7):

logðbÞ ¼ ln



   AEa Ea  5:331  1:052  RgðaÞ RT

ð7Þ

KAS method is another isoconversional integral method. This method is based on Eq. (8):

ln



b

T2



¼ ln



   AR Ea  gðaÞEa RT

ð8Þ

2.8. CFB gasifier The gasification tests were performed in a CFB gasifier pilot plant. The most significant parameters such as flows, temperatures and pressures were recorded by a data acquisition system. The unit was designed to operate at atmospheric pressure with air as fluidizing agent, not requiring any external heat source. The gasifier consists of a riser, which is a refractory steel type AISI 310 tube with a diameter of 300 mm in the bed section and has an overall height of 8764 mm, a cyclone that works as a gas solid separator and a recirculation system. The recirculation system consists in a standpipe, which is a tube of stainless steel with a diameter of 127 mm with a non-mechanical valve for solids flow control. The instrumentation of the plant includes a calibrated hopper for measuring feedstock flow rates, a calibrated thermal dispersion flow meter (ST50 FCI), k-type thermocouples for temperature measurements and pressure probes. The primary air, which is routed to a windbox that has a bubble cap distributor plate, is supplied through a blower. The producer gas was routed to a flame and only a sample of the gas was sent to the gas analyzer. Gas composition was determined using a Fourier Transform Infrared (FTIR) spectroscopy analyzer (Gasmet™ CX-Series FT_IR Gas Analyzer), a paramagnetic analyzer for O2 and a conductivity analyser for H2 (Sick-MAIHAK S710), a flame ionization detector (FID) for hydrocarbons detection and quantification (Heated FID-Analyzer 3-300 A) and a laser detector for H2S (LaserGas™ neo monitors as). Tar measurements were carried out through a tar sampling system (CEN/TS 15439), where a sample of producer gas was removed from the gasifier exit and driven to a series of impinger bottles where condense tars under low temperatures by use of a propan-2-ol extraction solvent. The tars retained in the solvent were detected and quantified by gas chromatography–mass spectrometry (GC–MS), by GC–MS (Aligent 6890/5975B with HP5

column). Also, the determination of volatile compounds such as toluene and benzene was done by GC–MS (HP 5890/5971A with CP-Sil8CB column). 3. Results and discussion 3.1. Raw wastes characterization The characterization was done by proximate and ultimate analysis of raw wastes. All data were collected during a full work day during, thus average values are presented in Table 2. In addition, the calorific value was determined to investigate its potential use as fuel for thermal conversion processes. From the results in Table 2, it can be observed that rejects showed a gross calorific value (GCV) of 26.6 MJ/kg, a high volatiles content of 82.5% and a medium ash content around 9%. This mean that rejects may be suitable to produce an acceptable fuel. Moreover, chlorine content of the rejects was significant, that could derived in the formation of dioxins and furans (Lehtikangas, 2001) when the fuel was used for thermal conversion. Regarding de-inking sludge, it presented a GCV of 7.6 MJ/kg and a high ash content. It could result on a great restriction of the amount of de-inking sludge to add because of the need to set up an ash removal system. Volatile content is an empirical measurement, which is used for coal, biomass and other fuels with a low content of ash, for determining the amount of volatile compounds released when a fuel is heated without air. When ash content is very high, a high amount of ash inorganic compounds are volatilized as CO2 from carbonates and alkaline compounds (KCl melts about 775 °C). Therefore, volatile content, which was about 48.4%, is overestimated and it is not an accurate measurement. As a consequence of the high proportion of ash found in the sludge (71.7%), the sum of C, H and O is low and thus, decreasing its calorific value. The major elements in the sludge ash was calcium (28% d.b. on ash basis or 20% on biomass basis), followed by silicon and aluminum. On the other hand, rejects sample has a high proportion of C, H and O, generating an important NCV about 28 MJ/kg. Chlorine content was significant, being about 1.34%. This chlorine is mainly organic chlorine and it comes from plastic materials due to ash forming elements have a low content of potassium and sodium when combined with chlorine produce KCl and NaCl. The major elements in the rejects sample are aluminum (24% on ash basis or 21% on biomass basis), followed by calcium and silicon. According to Directive 2009/28/EC, the biodegradable fraction of wastes can be considered as renewable source of energy. The determination of biomass content is of great importance because

Table 2 Proximate and ultimate analysis and GCV of raw wastes.

a b

Rejects

De-inking sludge

Proximate analysis (wt.%) Moisture Volatiles (d.b.)a Fixed carbon (d.b.) Ash (d.b.) GCV (MJ/kg, d.b.)

40.4 82.5 8.2 9.3 26.61

32.5 48.4 n.a. 64.6 7.57

Ultimate analysis (wt.%, d. b.) Carbon Hydrogen Oxygenb Nitrogen Sulfur Chlorine

58.3 8.4 22.26 0.29 0.13 1.34

25.6 2.7 6.35 0.43 0.25 0.01

(d.b.) dry basis of material. By difference.

Please cite this article in press as: Arenales Rivera, J., et al. Thermal degradation of paper industry wastes from a recovered paper mill using TGA. Characterization and gasification test. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.04.031

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J. Arenales Rivera et al. / Waste Management xxx (2015) xxx–xxx Table 3 Determination of biomass content of rejects.

Selective dissolution method Manual separation

Biomass

Non biomass

Inert

18 17

77 76

5 7

of allowing the estimation of the biodegradable fraction of wastes (Séverin et al., 2010). From Table 3, it is observed that the biomass content of rejects was about 17% and 18%, while the non-biomass fraction was between 76% and 77%. Hence, the addition of de-inking sludge may increase the biomass content. 3.2. TGA of raw wastes Fig. 1 shows TG and differential thermogravimetric (DTG) curves for the mixture of rejects that belong to the classification of the paper recovered from SC and rejects derived from the pulper. The curves presented here were obtained from experimental data at 20 °C/min from room temperature to 900 °C. The plot shows that the thermal decomposition occurred in two steps as it is observed from the DTG curve by the presence of two peaks. The first step corresponds to cellulosic materials, whose thermal degradation began at about 270 °C and was essentially completed by approximately 380 °C, presenting a peak at 320 °C. The second step may correspond to a mixture of plastic materials, whose thermal degradation started at 400 °C and ended at 520 °C, showing a peak at 495 °C. In this step, it can be seen two merged peaks that may correspond with different kinds of plastic materials. Although any qualitative composition analysis of plastic materials was done, a significant content of chlorine was measured that may correspond to PVC (Corella et al., 2008). Besides, low density polyethylene (LDPE) always is presented in large proportions, being a feasible source of energy (Shah et al., 2010), since is used in garbage bags and packaging of magazines, cardboard, etc. This is followed by a slow further loss of mass from 520 °C, which did not finish till 900 °C. The residue obtained was of 20% of the sample, which may consist of ash and unburned material.

The blends of rejects and de-inking sludge were produced with the addition of a small amount of de-inking sludge by mixing. Therefore, the contribution of de-inking sludge to the thermal behavior of each blend increased with the quantity of sludge added. Fig. 1 shows the TG/DTG curves at 20 °C/min from room temperature to 900 °C. From the plot, it is remarked that the thermal degradation happened in two different steps. The first degradation step started at about 250 °C and ended at approximately 430 °C, showing a peak at 350 °C that belongs to cellulosic materials (Mangut et al., 2006). This step is followed by a second degradation step, whose range varied from 680 to 850 °C when the degradation process was almost finished due to there was practically no further loss of mass. This step showed a peak at 750 °C due to the inorganic materials present in form of additives and charges in the ink removed. The experiment was kept for several minutes at 900 °C and no variation of the weight in the sample was found, thus the residue obtained may correspond to ash content, being about 50%. 3.3. Fuel characterization Pelletizing was carried out in order to obtain a suitable fuel for being used in fluidization bed technologies for combustion or gasification processes. Samples were obtained from pellets prepared in the densification process, so that the fuel characterization was done on a fuel that could use in a thermal conversion process. The results of the characterization of these materials are given in Table 4, where proximate and ultimate analysis of blends, as well as GCV are presented. A simple codification was done in order to define the fuels before showing the characterization to achieve a better understanding. The blends are defined as follows: M1 (90% rejects and 5% de-inking sludge), M2 (85% rejects and 15% de-inking sludge), M3 (75% rejects and 25% of de-inking sludge) and M4 (65% rejects and 35% de-inking sludge). As a function of de-inking sludge added, it is shown a cutting down in the volatile matter content from 75.2% to 66.7%. Nevertheless, it is high enough for gasification proposal because of the blends showed a high volatile content. This mean that the solid products may be converted instantly into gaseous products. An increment of ash content was seen from 13.1% to 28.3%, which may imply the need to install a system for collecting ash. In addition, all fuels showed a low content of fixed carbon, which is common in fuels derived from wastes (Montané et al., 2013). The ultimate analysis showed a reduction of carbon and hydrogen content when these paper industry wastes, which are carbon-based, were mixed with a certain quantity of de-inking

Table 4 Proximate and ultimate analysis and GCV of fuels. M1

M2

M3

M4

3.2 75.2 11.7 13.1 26.62

2.9 71.4 10.2 18.4 24.12

3.6 67.5 6.0 26.5 21.46

2.0 66.7 5.0 28.3 20.21

Ultimate analysis (wt.%, d.b.) Carbon 55.6 Hydrogen 7.6 Oxygenb 19.72 Nitrogen 0.35 Sulfur 0.07 Chlorine 1.56

50.9 6.9 21.78 0.39 0.08 1.55

48.7 6.8 16.14 0.26 0.15 1.45

44.9 6.0 19.18 0.32 0.13 1.17

Proximate analysis (wt.%) Moisture Volatiles (d.b.)a Fixed carbon (d.b.) Ash (d.b.) GCV d.b. (MJ/kg)

Fig. 1. TG and DTG curves of de-inking sludge and rejects at a heating value of 20 °C/min.

a b

(d.b.) dry basis of material. By difference.

Please cite this article in press as: Arenales Rivera, J., et al. Thermal degradation of paper industry wastes from a recovered paper mill using TGA. Characterization and gasification test. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.04.031

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J. Arenales Rivera et al. / Waste Management xxx (2015) xxx–xxx

sludge. As a consequence of the increment in proportion of ash due to sludge addition, the sum of C, H and O is reduced and thus, decreasing calorific value. Nevertheless, each fuel presented an important GCV, whose values were among 20.21 and 26.62 MJ/kg. Therefore, the mixtures composed mainly of rejects from recovered paper sorting and pulping process may be considered as promising fuels. The major elements in the fuels ash were calcium, silicon and aluminum. However, the addition of de-inking sludge changed its proportion in each blend. In regard to calcium, an increment was detected with the increase of sludge, from 24% to 40% d.b. on ash basis. However, aluminum and silicon underwent a sharp reduction with the addition of sludge. Silicon was reduced from 25% to 13% d.b., while aluminum was cutting down from 22% to 9.9% d.b. on ash basis. Regarding chlorine content, it was about 1.5%, being quite similar to rejects. No significant reduction of chlorine was detected with the addition of de-inking sludge. This fact could derived in the need of chlorine removal from the rejects because of the associated corrosion during the utilization of the combustible (Lehtikangas, 2001). As was remarked above the chlorine content may be due plastic materials, being as organic chlorine. Thus, the same conclusions can be extrapolated. Regarding de-inking sludge, the high amount of ash could cause operational problems related to the fusibility of the ashes with the bed material. Thus metal analysis content may be necessary to determine the rate between potassium/sodium and calcium/ magnesium content. Fuel ash was analyzed for oxides of the most abundant constituent elements, other than oxygen and sulfur: Si, Al, Fe, Ca, Mg, Mn, K, Na and Ti, see Fig. 2. Besides, other elements such as Ba, P, Sr and Zn were analyzed as well. The test was performed thrice to solve the problems related to the lack of homogeneity of the fuels and the small amount used for preparing samples for analysis (Hilber et al., 2007). The oxides of major elements in ash showed a high content in calcium oxide, typically due to the rejects are composed of cellulosic materials as some remaining fractions show (Arena and Di Gregorio, 2014). Its content increased with the addition of de-inking sludge from 24% to 40%, and also, magnesium content increased from 1.1% to 1.9%. Hence, there are a high content in Ca and Mg that may avoid the risk of sintering. On the other hand, the sodium content changed from 2.5% to 0.54% and the potassium content changed from 1.8% to 0.54%, being reduced in both cases with the amount of de-inking sludge added. The results of sodium and potassium content were no significant to consider a real risk of sintering with this type of fuels. In addition, the characteristic temperatures obtained in the ash melting behavior analysis (Dunnu et al., 2010) were quite high what reduce the risk of sintering as well, see Table 5. According to standard method (CEN/TR 15404), characteristic temperatures of the blends were analyzed, as it has been shown in Table 5. Firstly, the ashes were moulded into a cylindrical shape

Table 5 Characteristic temperatures for ash melting behavior of fuels. Temperature (°C)

M1

M2

M3

M4

Shrinking Deformation Hemisphere Flow

1260 n.a. n.a. >1400

1340 n.a. n.a. >1400

1210 n.a. n.a. >1400

940 n.a. n.a. >1400

and the sample was heated at 5 °C/min from 550 to 1400 °C. Secondly, the shrinking temperature, which is defined as the temperature at which the area of the sample falls below 95%, was measured. The shrinking temperature was quite high, being among 940 and 1340 °C for all fuels, in comparison with the operation temperatures in a fluidized bed gasifier. On the other hand, the deformation temperature and the hemisphere temperature could not be detected because of the fast change in the shape of the sample. Finally, the flow temperature, which is defined as the temperature at which the ash is spread out in a layer, was reached at a temperature up to 1400 °C.

3.3.1. Effect of addition of de-inking sludge in the biomass content The biomass content of the blends was determined in order to estimate how the de-inking sludge may increase the biodegradable fraction. Fuels with high biomass content are susceptible to be considered as renewable sources of energy (Dunnu et al., 2010). In Table 6, it has been presented the biomass content of the blends using the selective dissolution method. It can be observed an increment in the biomass content from 18% to 25% with the addition of de-inking sludge. On the other hand, the non-biomass fraction is reduced from 73% to 47%. Hence, the addition of de-inking sludge produce an increase of biomass content and a cutting down of non-biomass content.

3.3.2. Solid recovered fuel (SRF) classification Nowadays, new European laws have been developed in regard to fuels derived from wastes (EN 15357). The main requirement of these fuels, known as SRF, is being produced from non-hazardous wastes. A classification system for this type of fuels (EN 15359) has been developed. The classification has been established based on three properties: NCV, which is the economic indicator, chlorine content, which is an operational indicator, and mercury content, which is an environmental indicator. SRF can be produced from municipal, commercial and industrial wastes. Hence, paper industry wastes may be used as raw material for producing these secondary fuels due to its condition of non-hazardous wastes. Although these fuels were produced as RDF, a reliable classification as SRF was done. All fuels were analyzed to determine NCV, chlorine content and mercury content as the normative explains. M1 was classified as follows: NCV 2 (24.99 MJ/kg), Cl 5 (1.56%) and Hg 1 (0.002 mg/MJ). Regarding to M2, NCV 2 (22.64 MJ/kg), Cl 5 (1.55%) and Hg 1 (0.002 mg/MJ). M3 showed the following classification: NCV 2 (20.00 MJ/kg), Cl 4 (1.45%) and Hg 1 (0.002 mg/MJ). Finally, M4 presented the next classification: NCV 3 (18.92 MJ/kg), Cl 4 (1.17%) and Hg 1 (0.004 mg/MJ).

Table 6 Biomass content of fuels using the selective dissolution method.

Fig. 2. Elements in the ash (%).

Biomass Non biomass Inert

M1

M2

M3

M4

18 73 9

20 62 18

22 55 23

25 47 28

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J. Arenales Rivera et al. / Waste Management xxx (2015) xxx–xxx

3.4. Fuel preparation

Table 8 Physical properties of fuels.

3.4.1. Energy consumption during milling and pelletizing Energy consumption was recorded during milling and pelletizing. From Table 7, it can be observed the energy consumption by means of the specific mass flow and specific energy. The energy consumption during milling refers only to the energy used to reach a size reduction of 8 mm. Regarding pelletizing, selection of the die compression, which was 40 mm, is of great importance for obtaining pellets with acceptable physical characteristics. The tests included the preparation of a small amount of pellets for each blend chosen. Even though any further conclusion may be extrapolated due to the small quantity of pellets prepared, the values of the energy consumption obtained were in the same order of magnitude. Although the energy demand in size reduction was particularly important, the previous grinding was not taking into account for estimating the energy consumption of milling process. The consumption during milling was about 245 and 277 kW h/t. It could be explained because these materials are composed mainly of plastics and paper wastes. Paper and cardboard are rich in fibrous materials what could reduce the velocity of milling operations and increase the specific energy (Mediavilla Ruiz et al., 2007). According to energy consumption in pelletizing, the amount of de-inking sludge added did not cause an increase in consumption, being between 67 and 84 kW h/t. However, the specific mass flow in pelletizing was higher compared to milling process, being around 5 kg/h kW. 3.4.2. Quality of pellets Not only the energy consumption was investigated during pelletizing, but also the quality of fuels was evaluated. Pellet quality was assessed based on the estimation of several physical parameters such as moisture, durability, fines and bulk density. Hence, the combination of both investigations resulted in the optimization of the densification process. 3.4.3. Effect of de-inking sludge in the bulk density and durability of fuels Table 8 presents the most important physical parameters of fuels. All analysis were carried out trying to maintain the same moisture content avoiding differences due to the effect of moisture in the physical properties. Regarding raw wastes, the bulk density was 780 kg/m3 for de-inking sludge and 100 kg/m3 for rejects, as they were received from the paper mill. The bulk density increased with the amount of de-inking sludge added, which agrees with the bulk densities of raw wastes The maximum value of bulk density (510 kg/m3)was reached at 35% of de-inking sludge added. Regardless of the amount of de-inking sludge added, the durability remains constant, around 98%. Besides, the percentage of fines determined during the tests showed a slightly increment as the amount of de-inking sludge increased.

Moisture (%) Durability (%) Bulk density (kg/m3) Fines (%)

M1

M2

M3

M4

3.2 98.6 420 0.7

2.9 98.2 430 0.8

3.6 98.2 460 0.9

2.0 98.2 510 1.0

choice was done because rejects can be easily dewatered what could influence the tests, while sludge is able to maintain moisture content. It can be observed from Fig. 3 that the durability decreased sharply when the moisture content was up to 10%. On the other hand, the bulk density decreased slightly when the moisture reached a value of 15%. 3.5. TGA of fuels The survey was carried out with the purpose of knowing the thermal behavior of the blends of rejects and de-inking sludge. The characterization of the fuels was completed with TGA, which also allowed the determination of the activation energy for each degradation step. The TG/DTG curves of the blends at 20 °C/min have been presented in Fig. 4. DTG curves were obtained for each heating range, from 5 to 20 °C/min. The determination of the thermal degradation range of each blend is complicated because of the overlapping of peaks of each original material. In addition, the domain of degradation of rejects compared with the de-inking sludge in the DTG curves is noted through the size of the peaks. Therefore, the temperature range of the blends changes slightly in comparison with those for each individual component. In the blends, rejects and de-inking sludge degrade at different temperatures than pure residues, although the variation is not significant. DTG curves for each blend at 20 °C/min are shown in Fig. 4. Three peaks can be observed that belong with three degradation steps. The first degradation step which corresponds to the decomposition of cellulosic materials, started at 230 °C and was completed at 380 °C. The second degradation step, determined in the range from 380 to 550 °C, is attributed to the decomposition of plastic materials (Cai et al., 2007). Finally, the third step, which was obtained from 640 to 780 °C, may belong to the decomposition of the inorganic additives retained by the sludge. Other studies on RDF showed a composition mainly based on cellulosic materials, wood and plastics such as polypropylene, and polyethylene (Cozzani et al., 1995). The temperature range and the maximum temperature values of each degradation step are shown in Table 9. The peak temperature of each degradation step was moved slightly on the right when the heating rate increased. Although, the values of the peak temperatures were quite similar, small variations were detected that could be originated because of the heterogeneity of the material.

3.4.4. Effect of moisture content in bulk density and durability The effect of moisture content in the physical properties (Ruiz Celma et al., 2012) was studied through the analysis of M3. This Table 7 Energy consumption of fuels in milling and pelletizing. M1

M2

M3

M4

Milling (dry basis) Specific mass flow (kg/h kW) Specific energy (kW h/t)

2.2 277

2.7 245

2.3 268

2.6 270

Pelletizing (dry basis) Specific mass flow (kg/h kW) Specific energy (kW h/t)

5.0 67

4.7 84

5.1 77

5.2 71

Fig. 3. Effect of moisture content in bulk density and durability of M3.

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J. Arenales Rivera et al. / Waste Management xxx (2015) xxx–xxx

Fig. 4. TG curves of rejects/de-inking sludge mixtures at a heating rate of 20 °C/min. Inset: DTG curves.

3.6. Kinetic study 3.6.1. Kinetics of raw wastes The determination of the kinetics parameters for rejects was carried out under Coats and Redfern method, see Fig. 5. The temperature range during the degradation of the sample was used to obtain the main degradation steps, whose peaks were at about 320 °C and 495 °C. The values of activation energy, pre-exponential factor and reaction order for cellulosic materials degradation were 88.4 kJ/mol, 7.2  106 min1 and 4. The second step of degradation, which belongs to plastic materials, showed the following values: 184.7 kJ/mol, 9  1012 min1 and 1.5, respectively. The kinetics parameters for de-inking sludge were obtained under the same method used for rejects analysis in order to compare the results, see Fig. 5. The first step of degradation showed a peak that may belong to some kind of cellulosic materials from retained fibres during the de-inking process. The values of activation energy, pre-exponential factor and reaction order for this step were 45.4 kJ/mol, 6  102 min1 and 2.5, respectively. The second step of degradation was referred to the additives, which showed a peak at approximately 700 °C, being the kinetics parameters 164.4 kJ/mol, 1.9  108 min1 and 1.1.

Fig. 5. Coats and Redfern method: (A) rejects (B) de-inking sludge.

3.6.2. Kinetics of fuels The estimation of the kinetic parameters was carried out under three different non-isothermal methods. These samples showed an important contribution of plastic materials considering the peaks found in the TG/DTG curves. Coats and Redfern method was used

in order to compare the activation energy and the exponential factor with raw wastes. The blends showed three main steps of degradation, thus three activation energies were determined. The values of the activation energy were closed with the raw materials, except for the last step of degradation, which is related to additives in the sludge. It was determined a first peak that corresponds to the peak detected in the rejects due to cellulosic materials, with activation energies about 60–80 kJ/mol. Moreover, a second peak was detected, that is assumed to be the peak of plastic content in the rejects, with activation energies around 120–160 kJ/mol, see Table 10. The other non-isothermal methods used were FOW and KAS. Both are isoconversional integral methods, thus an activation

Table 9 Temperaure range of the pyrolysis and maximum temperature values of fuels.

Table 10 Kinetic parameters of fuels by Coats and Redfern method.

5 °C/min

10 °C/min

20 °C/min

207–359 °C (284.1 °C) 386–510 °C (461.2 °C) 598–711 °C (667.2 °C)

216–368 °C (287.1 °C) 377–530 °C (474.1 °C) 641–728 °C (694 °C)

239–376 °C (312.2 °C) 376–539 °C (483.9 °C) 654–751 °C (698.1 °C)

238–328 °C (278.8 °C) 402–529 °C (468.1 °C) 661–710 °C (693.5 °C)

247–377 °C (284.3 °C) 384–529 °C (473.3 °C) 644–743 °C (708.1 °C)

257–384 °C (311 °C) 393–548 °C (491.6 °C) 644–777 °C (732.9 °C)

222–354 °C (327.7 °C) 376–494 °C (459 °C) 607–719 °C (684.6 °C)

248–385 °C (349.4 °C) 385–526 °C (472.6 °C) 630–759 °C (684.6 °C)

248–384 °C (311.3 °C) 384–538 °C (492.3 °C) 645–787 °C (684.6 °C)

152–372 °C (328.1 °C) 354–501 °C (451.5 °C) 584–714 °C (688.3 °C)

170–376 °C (331.4 °C) 367–510 °C (465.1 °C) 599–759 °C (714.5 °C)

211–385 °C (348.5 °C) 385–538 °C (482.8 °C) 627–778 °C (741 °C)

M1

Ea (kJ/mol)

A (min1)

n

R2

73.6 141.4 47

4.8  105 7  109 1.5  102

2.5 2 1.1

0.994 0.989 0.987

78.9 161.9 41.6

1  106 1.4  1011 6  101

2.5 2 1.1

0.982 0.981 0.946

77.4 118.7 50.1

1.4  106 1.2  108 1.7  102

2.5 2 1.1

0.995 0.986 0.948

62.1 152.4 55.9

1.3  104 2.3  1010 3.1  102

2.5 1.5 1.1

0.997 0.991 0.943

M1

M2

M2

M3

M3

M4

M4

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3.7. Gasification tests 3.7.1. Fuel selected Only one of the blends was used for gasification tests. M1 was chosen due to the low amount of ash, so any ash removal system was necessary. Nevertheless, all the ash produced during the gasification process was collected in a cyclone placed at the end of the gasifier. M1 presented a high calorific value, being a promising fuel for thermal conversion (Aranda Usón et al., 2012). The energy consumption during pelletizing was recorded as it was done for the initial blends. The parameter used to follow the energy consumption was the specific energy. In that case, the whole process was investigated from grinding to pelletizing. Firstly, rejects were milled in a double bearing horizontal axis knife mill at 25 mm, which had a specific energy of 111 kW h/t. Secondly, rejects were mixed along with de-inking sludge and milled in a double bearing horizontal axis knife mill at 8 mm, being the specific energy of 228 kW h/t. After that, the moisture of the mixture was reduced in a solar dryer until it was suitable for pelletizing. The specific energy in the solar dryer and the pellet plant was of 12 kW h/t and 56 kW h/t, respectively. To sum up, the total specific energy for the fuel preparation process was 407 kW h/t.

Fig. 6. Estimation of activation energy using the FOW and KAS method.

energy may be estimated for each value of conversion. These model-free methods in comparison with the model-fitting methods, allow to estimate the activation energy as a function of the conversion instead of the assumption of a reaction model. An example plot of the isoconversional curves derived from the FOW and KAS methods has been shown in Fig. 6. These two methods were used to compare the values of activation energy obtained in the thermal degradation steps of the samples described above. As it was determined through TG/DTG curves, three degradation steps were detected, which correspond with three different components such as cellulosic materials, plastic materials and additives. Nevertheless, isoconversional models allow the determination of the activation energy for each value of conversion, which are related to different steps of degradation. Regarding these samples, the activation energy was calculated from 0.1 to 0.9 in order to cover the whole thermal degradation range, see Table 11. The degradation of plastics materials was located between 0.4 and 0.8 of conversion values, showing similar results with respect to other studies where the main components of RDF were analyzed (Lin et al., 1999). In regard to cellulosic materials, clearly identified from 0.1 < a < 0.2, the results presented a significant variation may be because of the heterogeneity of the samples. Table 11 Activation energy of fuels by FOW and KAS methods. Ea (kJ/mol)

M1

M2

M3

M4

Cellulosic materials FOW KAS

73.1 66

74.2 69.7

62 55

56.4 49.7

Plastic materials FOW KAS

169.5 164.4

162.9 160

160.4 154.7

161 157.3

3.7.2. Operation conditions The facility used in the gasification tests was a CFB pilot plant of 500 kWth operated at atmospheric pressure, see Fig. 7. Tests were carried out using silica sand as bed material and air as gasifying agent. Solids flow control was done through a U-type loop-seal non-mechanical valve, being complicated to establish smooth operation because of the particularly design of the valve and the attrition of bed material. First at all, the gasifier was heated by the introduction of a small amount of preheated air through the windbox. After reaching the ignition temperature of pellets, which is around 450 °C, a few amounts of pellets was fed to increase the temperature inside the gasifier under combustion conditions. Finally, the preheater was turned off when the gasifier reached a suitable temperature to maintain the gasification process without the help of any heat external source, as an industrial gasifier works. The feeding was increased until the operation conditions, which were selected before starting each test, were reached. Each test was performed in eight hours of stable operation without any operational problems to obtain experimental data. 3.7.3. Gasification performance Several gasification tests were done in order to demonstrate the feasibility of the valorisation of paper industry wastes prepared by means of pelletizing. Although some parameters were fixed for all tests as bed material used (silica sand of 428 lm) and total solids inventory (100 kg), the best operational conditions were determined during preliminary tests. The parameters analyzed during the tests were: temperatures and pressures around the loop, gas composition and tar content. The best performance was done at ER of 0.3. Feeding was kept at 100 kg/h during the whole experiment. The main results were: bed temperature of 850 °C, NCV of 5 MJ/Nm3 and total tar content of 11.44 g/Nm3. The gas composition of the producer gas resulted in 11.8% of carbon dioxide, 6.9% of carbon monoxide, 3.8% of hydrogen, 4.7% of methane and 2.9% of ethylene. This results are according with other studies using SRF fuels for valorisation in a fluidized bed gasifier (Dunnu et al., 2012). However, few studies were done using a CFB gasifier able to maintain isothermal conditions, using a fuel from wastes as feedstock (Granatstein, 2003). Most of investigation was carried out in allothermal units (Meng et al., 2011; Siedlecki and de Jong, 2011; van der Drift et al., 2001). None of sintering materials was detected after gasification tests and feeding was done without

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J. Arenales Rivera et al. / Waste Management xxx (2015) xxx–xxx

Fig. 7. Process flow diagram of the gasification process.

any problems, thus all gasification tests for this type of fuel were successful.

4. Conclusions Nowadays, new European laws as Directive 1999/31/EC has establish a hierarchy of treatment for reducing the volume of wastes send to disposal in landfill. Hence, the production of secondary fuels from industry wastes and its valorisation has been studied. In this paper, paper industry wastes, composed of rejects and de-inking sludge, were used as raw materials. Different blends were prepared with the addition of a certain amount of de-inking sludge to the rejects. Fuel characterization was carried out by chemical and physical analysis. TGA of raw wastes and blends was carried out at three heating rates in an inert atmosphere, simulating a pyrolysis process. TG/DTG curves showed clearly three steps of degradation. The first step, obtained in the temperature range (200–360 °C) corresponds to cellulosic materials. The second step occurs between 360 and 530 °C is attributed to plastic materials. The third step between 580 and 720 °C belongs to additives of de-inking sludge. The activation energies were estimated by the application of three non-isothermal methods (Coats-Redfern, KAS and FOW). In the first and second steps, activation energies 140–160 kJ/mol for cellulosic materials and 50–80 kJ/mol for plastic materials were obtained. As a result of fuel characterization, a blend consisted in 95% of rejects and 5% of de-inking sludge, was prepared for testing in a CFB gasifier pilot plant at atmospheric pressure under autothermal operation. The total energy consumption for fuel preparation process was 407 kW h/t. The most part of energy was used for grinding (339 kW h/t), while only 56 kW h/t were consumed in the pellet press. On the other hand, drying of raw wastes was carried out in a solar dryer with reduced energy consumption (12 kW h/t). The fuel was fed as pellets of 8 mm of diameter. Regarding physical properties, bulk density and durability were 420 kg/m3 and 98.6%. No bridge formation into the hoppers was seen during continuous operation of the gasifier. Regarding gasification tests, syngas quality was determined by means of NCV determination. The best performance was done at ER equals to 0.3 at 850 °C that yielded an energy content of 5 MJ/Nm3. Tar concentration based on GC–MS analysis showed a

value of 11.44 g/Nm3. On the other hand, no agglomerates were observed after inspections of bed and ash materials. Therefore, the valorisation of the secondary fuel produced for using in fluidized bed technologies was successful. As future work, gasification tests of each blend should be done to certify its use as a promising fuel for gasification process in fluidized bed. Although each blend has been classified according to EN 15359 standard, a more exhaustive investigation should be done. The new requirements to consider a processed waste as SRF, that promote their acceptability on a fuel market and increasing the public trust, are more restrictive compared with RDF requisites. Acknowledgements This research was supported by the Autonomous Community of Madrid (PROLIPAPEL II project P-2009/AMB-1480) and the authors are grateful for the financial support. References Aboulkas, A., El Harfi, K., El Bouadili, A., Nadifiyine, M., Benchanaa, M., Mokhlisse, A., 2009. Pyrolysis kinetics of olive residue/plastic mixtures by non-isothermal thermogravimetry. Fuel Process. Technol. 90 (5), 722–728. Ahmadi, B., Al-Khaja, W., 2001. Utilization of paper waste sludge in the building construction industry. Resour. Conserv. Recycl. 32 (2), 105–113. Aranda Usón, A., Ferreira, G., Zambrana Vásquez, D., Zabalza Bribián, I., Llera Sastresa, E., 2012. Estimation of the energy content of the residual fraction refused by MBT plants: a case study in Zaragoza’s MBT plant. J. Clean. Prod. 20 (1), 38–46. Arena, U., 2015. From waste-to-energy to waste-to-resources: the new role of thermal treatments of solid waste in the Recycling Society. Waste Manage. 37, 1–2. Arena, U., Di Gregorio, F., 2014. Energy generation by air gasification of two industrial plastic wastes in a pilot scale fluidized bed reactor. Energy 68, 735– 743. ASPAPEL, Association of Spanish Pulp, Paper and Cardboard Manufacturers, 2014. (accessed 17.06.14). Cai, J., Wang, Y., Zhou, L., Huang, Q., 2007. Thermogravimetric analysis and kinetics of coal/plastic blends during co-pyrolysis in nitrogen atmosphere. Fuel Process. Technol. 89 (1), 21–27. Ceamanos, J., Mastral, J.F., Millera, A., Aldea, M.E., 2002. Kinetics of pyrolisis of high density polyethylene. Comparison of isothermal and dynamic experiments. J. Anal. Appl. Pyrolysis 65 (2), 93–110. CEN/TR 15404. Solid Recovered Fuels – Methods for the Determination of Ash Melting Behaviour by Using Characteristic Temperatures. (accessed 01.08.14). CEN/TS 15401. Solid Recovered Fuels – Determination of Bulk Density. (accessed 01.08.14). CEN/TS 15439. Biomass Gasification – Tar and Particles in Product Gases-Sampling and Analysis. (accessed 01.08.14).

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Please cite this article in press as: Arenales Rivera, J., et al. Thermal degradation of paper industry wastes from a recovered paper mill using TGA. Characterization and gasification test. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.04.031