Performance improvement in olive stone's combustion from a previous carbonization transformation

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ “This is an Accepted Manuscript of an article published by Elsevier in: FUEL on 2018, available at: https://doi.org/10.1016/j.fuel.2018.04.127” 1 Performance improvement in olive stone's combustion from a previous carbonization transformation A. Gómez-Martín 1, 2,*, R. Chacartegui 3, J. Ramírez-Rico1, 2, J. Martínez-Fernández1, 2 1 Dpto. Física de la Materia Condensada, Universidad de Sevilla, Avda. Reina Mercedes SN, 41012 Sevilla, Spain. 2 Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla), Avda. Américo Vespucio 49, 41092 Sevilla, Spain. 3 Dpto. de Ingeniería Energética, Universidad de Sevilla, Camino de los Descubrimientos SN, 41092 Sevilla, Spain. * Corresponding author. Tel. +34 954556423 email: [email protected] Abstract Under the framework of circular economy, agricultural wastes are an interesting carbon-based feedstock for thermal energy and power generation. Their use could extend the availability of biomass-based fuel and at the same time would reduce negative environmental effects. However, and depending on the residues’ characteristics, their direct combustion in boilers presents some challenges which could be overcome with a carbonization pre-treatment. In this paper, the main mechanisms of thermochemical transformation of an abundant agricultural waste, olive stone, into biochar products via slow carbonization are analyzed, with emphasis on the effect of peak carbonization temperature. Thermogravimetric and differential scanning calorimetry analysis are used to evaluate the performance of the resulting biochars compared to raw olive stone in combustion processes and to assess the correlation between the peak carbonization temperature and compositional and fuel properties Results show that with a prior treatment up to an optimum temperature of 800ºC the energy density is increased up to three times compared to the raw material. These findings suggest that carbonization of olive stones reduces the barriers to their direct use in current biomass boiler technology. Keywords: olive stone, agricultural waste, biochar, energy conversion, combustion 2 Highlights: • Carbonization of olive stone stones leads to improved fuel properties • There is a three-fold increase in heating value after carbonization at 800 ºC • Higher pretreatment temperatures penalizes combustion performance • Olive stone biochar capacity as alternative fuel for industrial use is shown 1 Introduction The use of renewable biomass and agricultural residue sources for thermal energy and power generation are of high interest for a sustainable development owing to their CO2 neutral emissions, worldwide abundance, selfsufficiency and relatively low cost [1, 2]. Biomass precursors are the focus of widespread attention as starting material in a wide variety of thermo-chemical energy production processes such as combustion, gasification and pyrolysis [3]. Olive oil extraction and pitted olive production represent two of the most relevant industries within the Mediterranean countries [4, 5]. These industries can in turn be regarded as one of the largest producers of biomass wastes (either in the liquid form usually referred to as olive-mill wastewater, or solids residues such as dried pomace, olive stone, olive tree pruning and leaves [6]). Among these wastes, the overall estimated production of olive stones in Spain is approximately 1,050,000–1,400,000 tons per year (campaign of 2017) [7]. Given the abundance and low cost of this byproduct, there is interest in exploring new uses for olive pit biomass [8-10]. Nonetheless, its main use so far has been as direct solid biofuel for in domestic applications [11] because, despite the environmental benefits and interesting physical and chemical properties [12], direct combustion has some drawbacks in commercial industrial boilers. Among them are the high moisture content, the low energy density, the limited fixed carbon fraction and problems with storage and handling costs. There are research lines aimed at upgrading the fuel properties of biomass through a feasible previous thermochemical process including torrefaction, slow pyrolysis or carbonization and hydrothermal carbonization. These processes apply mild energy in the absence of oxygen and thermally decompose the initial biomass material into syngas, bio-oil and a solid biochar as target products, which can serve as an 3 improved direct source for energy conversion and compete, in terms of energy density, with lignite and bituminous coals when environmental impact is taken into account [5, 13-17]. The difference between these processes mainly lies in the temperatures considered: torrefaction is usually performed at 200-350ºC, while in the case of slow pyrolysis or carbonization temperatures above 600-700ºC are frequently considered. The effect of parameters such as peak temperature [18-20], heat rate [21], pressure or purge gas [22], starting raw material [23], and residence time [24, 25] on biochar’s fuel properties have been previously studied in order to select the right operating conditions to the design of combustion equipment. According to the work of Lee et al. [24], the energy yield of torrefied mixed softwood showed a stronger dependence on reaction time than on the peak temperature, which was studied below 270ºC. However, in later representative works of Liao et al. [26], Lee et al. [27] and Mundike et al. [21], it was reported for different feedstock precursors that in slow pyrolysis processes with peak temperatures from 300 to 700 ºC, the temperature had the greatest influence in the resulting biochar’s characteristics, showing a strong drop in the solid yield but an increase in energy content. In terms of maximum temperature, some works reported a progressive increase in energy content within the range of 300-600ºC [28, 29], while others reported a maximum peak at temperatures above 550- 600 ºC followed by a slightly decrease [21]. Overall, the yield of bio-char decreased with increasing temperature but it is strongly influenced by the cellulose, hemicellulose and lignin contents [23, 30]. Prior work on the use of olive-pits as fuel has been devoted to its properties in direct combustion [31], its pyrolysis characteristics [32], the use of thermal analysis for the determination of combustion and pyrolysis kinetics [7, 33]. Only a few recent works have obtained biochars from pre-thermochemical transformation (mostly from slow carbonization or torrefaction) and correlated their treatment temperature with the improved fuel properties in direct combustion [34-39]. Most of the previously published works in this topic investigate pre-treatment temperatures below 900ºC, so the tendency and combustion characteristics of samples treated at higher temperatures is not completely understood. If a greater understanding of chemical reactions and parameter dependence developed in these processes is achieved, they could be modeled and designed for achieving an optimum efficiency when modeled at an industrial scale. Within this framework the main aim of this work is to explore the use of an abundant agricultural waste such as olive stone as starting precursor in the preparation of carbon-rich biochar samples through a carbonization pre-treatment in a wide range of temperatures from 400 to 1400ºC, and then optimize and understand the related thermal combustion and physical characteristics. 4 The structure of the paper is the following. First, a complete experimental characterization of this raw agroindustrial waste is herein performed in order to evaluate its energy potential. Then, the study is focused on a thorough thermal analysis of the olive stone carbonization process, obtaining carbon samples at different maximum temperatures. Afterwards, thermal characteristics of the samples are examined under oxidative atmospheres by means of thermogravimetric and differential scanning calorimetry analysis. Furthermore, the effect of the peak temperature on the combustion efficiency as well as combustion and microstructural characteristics is evaluated. 2 Materials and Methods 2.1 Raw material and carbonization process In this work an agricultural by-product, olive stone [40], was selected as starting material and subsequently carbonized in an inert atmosphere. The selected biomasses sources are solid wastes from the olive-oil extraction process, a procedure where the virgin olive oil is separated from the remaining components. Samples were supplied by Oleomoron S.L Corporation, an olive-oil extraction company located in the south of Spain. Prior to pyrolysis, a complete material characterization was performed in order to evaluate its suitability as alternative fuel, including studies about thermal behavior and compositional analysis. For this purpose, raw materials as received were first naturally air-dried and were then ground into particles with regular size (4 - 5 mm in diameter) to avoid heterogeneities. Carbonization was then carried out in a tube furnace under an inert atmosphere. Olive stone samples were submitted under a flow rate of nitrogen to a heating rate of 1 °C·min-1 up to 500°C and 5°C·min-1 up to the peak temperature followed by a holding time of 30 minutes. This temperature program was chosen to avoid cracks formation due to rapid release of volatile matter content upon heating. Peak carbonization temperatures ranged from 400 °C to 1400 °C in intervals of 200°C. Therefore, six types of biochar samples were obtained with peak temperatures of 400, 600, 800, 1000, 1200 and 1400°C. These temperatures were chosen to study the influence of this parameter on microstructural and thermal behavior. 5 Throughout the text, biochar samples will be referred as CO (Char from Olives) followed by the peak temperature used during the previous carbonization. 2.2 Characterization 2.2.1 Thermal analysis The thermal behavior of olive stones and biochar-derived samples was characterized by thermogravimetric (TGA) and differential scanning calorimetry (DSC) experiments. Nitrogen and synthetic air at flow rates of 100 ml·min-1 were used as inert and oxidizing carrier gases to study carbonization and combustion processes, respectively. Measurements were carried out using a dual system that allowed the simultaneous monitoring of weight loss and the heat flow as a function of the temperature increase (Thermal Advantage SDT Q-600). Differential thermogravimetric (DTG) curves were obtained as the result of the derivative of weight loss as a function of temperature. Calibration tests were periodically performed using a standard sapphire sample. For each experiment, samples weighing in the range 15-20 mg were placed in alumina crucibles and were then heated up from room temperature to 1000°C at constant heating rates of 10ºC·min-1. The initial mass for each of the tests was kept between 15 and 20 mg in order to avoid any additional effect associated to mass and heat transfer as well as to reduce the occurrence of secondary vapor solid interactions [41]. Three experiments at each set of experimental conditions were carried out to ensure the reproducibility of results. Heating value (HV) was estimated by integrating the heat flow curve with respect to time, linking it with the peak temperature of the previous carbonization process. For that, DSC curves were previously corrected with baseline curves obtained from experiments performed with empty crucibles at the same conditions. The ignition temperature (Ti) was defined as the temperature at which the first deviation from the baseline line is observed. The burnout temperature (Tf) was considered as the temperature at which the combustion process is almost completed. The energy conversion efficiency (ηbiochar) of the biochars was calculated on the basis of the heating value of the raw material and biochars as well as the biochar yield [ηbiochar (%) = (HVbiochar · biochar yield (%)) / (HVraw material)] [29]. 2.2.2 Proximate analysis From thermogravimetric experiments, the proximate analysis of raw olive stones and biochars was estimated taking into account the ash, volatile matter and fixed carbon content. The volatile matter (VM) was determined as the mass released during heat treatment up to 900°C under inert atmosphere including moisture content (average value of three measurements) [42]. Ash content was established as the remaining mass at 600ºC after 6 the complete combustion under air flow. The fixed carbon content (FC) was thus obtained by difference between the ash and volatile matter content [43] [FC (%) = 100 – (Ash+VM)]. The biochar yield at each peak temperature was calculated taking into account the mass remaining at this temperature after carbonization of the raw material by TGA measurements (Solid yield (%) = biochar weight/raw material weight x 100). Chemical composition of ashes obtained from combustion of olive stones up to 600ºC was analyzed by inductively coupled plasma mass spectrometry (ICP-MS, Ultima 2, Horiba Jobin Yvon). Samples were first digested in nitric acid using a Milestone ETHOS ONE microwave and were then diluted to ensure their concentrations fell within the detection limits of the instrument. 2.2.3 Peak carbonization temperature dependence Evolution of real and apparent density in carbon samples was determined by helium pycnometry (Pentapycnometer 5200, Quantachrome Instruments) and geometrical measurements, respectively. Scanning electron microscopies (Jeol 6460-LV and Hitachi S-5200) were used for microstructural characterization. Energy dispersive X-ray analysis (EDX- Inca 7573, Oxford Instruments) was used to identify chemical composition. Trace components presented were determined by an X-Ray fluorescence spectrometer (Axios, PANalytical) equipped with a Rhodium X-ray tube. Measurements were carried out on disks, which were pressed in a hand press (Retsch) over a mold of boric acid, using a semi-quantitative method (Omnian-10). The specific surface area was determined by nitrogen adsorption at 77K according to the BET method (ASAP 2420, Micromeritics). Samples were previously outgassed at 400°C during 10 hours and then were placed in a Dewar vessel to analyze the adsorbed and desorbed nitrogen amount while the relative pressure (P/P0) was being ranged from 0 to 1. All these properties were studied as a function of the peak carbonization temperature. 3 Results and Discussion 3.1 Raw olive stones 3.1.1 Thermal analysis The thermochemical properties of the carbonization and combustion processes of the raw material were analyzed using TGA measurements in controlled atmospheres of nitrogen and air, respectively. In Fig. 1 are 7 illustrated the weight loss percentage and the weight loss rate as a function of the temperature in both carbonization (Fig.1-A) and combustion (Fig.1-B). According to the results, as depicted in Fig.1-A, when the olive stone is submitted to a controlled heating rate under inert atmosphere, the sample is subjected to a thermal degradation [44]. On the DTG curve, three welldefined main stages can be distinguished [45]. The first one small peak, which takes place at temperatures up to 200°C, corresponds to the release of water molecules absorbed into the structure of the raw material. This step is followed by two abrupt peaks (with maximum weight loss rates at 275°C and 350°C in the DTG curve). A prolonged heating at temperatures above 200°C in non-oxidizing atmosphere causes the decomposition and break down of the main polysaccharide constituents present in lignocellulosic materials: hemicellulose, cellulose and lignin [46]. The polymer decomposition of hemicellulose happens at a temperature range of approximately 220-315°C, whereas the breakdown of cellulose takes place from 320ºC to 400ºC. Meanwhile, the decomposition range of the lignin is not as narrow as that of hemicellulose and cellulose. Its decomposition rate increases slowly over a broad range, with most of the decomposition occurring from 220ºC to almost 700°C [47- 49]. From 700ºC onwards, the carbonization process is almost complete as the bulk of volatile matter has been released, leaving a solid carbonaceous residue with around 25% of the initial mass. Nevertheless, when olive stone is heated under air, four main stages are observed due to the heterogeneous combustion of the fixed carbon content also taking concurrently with thermal degradation [50]. This behavior is depicted in Fig.1-B, where the first three stages are coincident with the results reported in inert atmosphere, while the last peak corresponds to the burning of the resulting char in air after the release of volatile matter content. As has been suggested in previous works, the temperature ranges and weight loss percentages of these processes in both inert and oxidative conditions can be substantially affected by parameters such as the sample mass, the particle geometry, chemical differences in the precursor and heating rate [51]. Since differential scanning calorimetry analysis is an essential technique for better understanding the suitability of a material as fuel for subsequent applications in combustion processes [52], Fig. 2 shows the DSC curves and the amount of heat absorbed or released while the temperature is increasing up to 1000°C under inert and oxidizing flowing gases. The carbonization process is mainly endothermic, as energy is required to break the polysaccharide chains, whereas the combustion process of the olive stone is overall highly exothermic. A pronounced peak of released heat at temperatures between 300 and 600°C is associated with char oxidation. The fixed carbon content of the sample burns out leaving ashes that consist primarily of metal oxides. At 600° C the combustion is almost completed, with a remaining ash content below 1 wt. %. 8 3.1.2 Proximate analysis Proximate analysis of the raw biomass is listed in Table 1. As shown there, the olive stone shows a moisture content lower than 10 wt. %, which is considered the optimum percentage for biomass combustion [53]. Higher moisture content might adversely affect the heating value causing ignition problems, releasing large amounts of flue gases and worsening the quality of combustion. In addition the remaining ash yield is generally below 1.0 wt. %, a lower percentage than other biomass products which generally show values ranging from 5% to 20% [54]. By comparing volatile matter and ash content of olive stones with those reported in previous works [8, 54], the volatile matter had values close to 70% and an ash content close to 1 wt. % although these values are usually rather dependent on growing conditions and harvesting [29]. 3.1.3 Chemical properties of the ashes The ash content and composition are relevant parameters to assess the potentially use of a biomass product as a fuel due to the problems that it might cause: severe slag-ash agglomeration, deposits on the surface of a heat exchanger or in a boiler tube, corrosion problems in the metal and ash emissions in the aerosol state (flyash) [55]. Furthermore, a high ash percentage reduces the heating value of a fuel and requires the use of dust removal systems [53]. Major elements in biomass ashes include Ca, K, Al, Si, Fe, Mg, Na, Cl, P, and S , although the amount of each element can be influenced by different factors such as the geographical location, the type of soil or the age [55]. The composition could either be inherent to the precursor or could be modified as a result of processing steps. For instance, salts are frequently bonded chemically to the organic fraction or can be incorporated during growth. During the combustion, the inorganic fraction of the biomass and the ashes undergo transformations such as segregation, precipitation and nucleation, depending on the combustion temperature, residence time and boiler design. Taking into account the combustion problems, one of the most important features of the ash is its melting point. The alkali oxides and salts that form during the combustion of the biomass have low melting points, and higher amounts of potassium and sodium can enhance problems with eventual slagging and fouling in the furnaces [56]. Another major concern related to ashes is the presence of both chlorine and alkali metals in some biomass products. For instance, potassium chloride presents high thermal stability and causes corrosion problems in heater tubes. Table 2 shows the main composition of ashes remaining after the combustion of olive stones. As can be seen, the ash composition is mostly dominated by metal oxides of Ca and K. Small traces of Mg, Fe and P are also 15 Figure 4. Evolution of volatile matter content, fixed carbon content and biochar yields as a function of peak temperature. Figure 5. (A) Real density, apparent density, (B) BET surface area and micropore volume evolution as a function of carbonization temperature of olive stones. 16 Figure 6. TGA (A) and DSC (B) representative curves under oxidizing conditions. Samples of raw olive stone and biochar derived from its carbonization at different peak temperatures were heat treated up to 800ºC with a constant heating rate of 20°C·min-1 in flowing air. 0200 400 600 800 1000 1200 1400 5 10 15 20 25 CO - 1200 ºC CO - 1400 ºC CO - 1000 ºC CO - 600 ºC CO - 800 ºC CO - 400ºC Heating value (kJ/g) Carbonization Temperature (ºC) Raw olive stone Figure 7. Heating value of combustion via DSC analysis as a function of the peak pre-treatment temperature. The integrated heat released was taken as the average value from three experiments. Errors represent one standard deviation. The solid is included to guide the eye and does not imply a fit to the data. 17 400 600 800 1000 1200 1400 45 50 55 60 65 70 Energy conversion efficiency, hbiochar (%) Carbonization Temperature , Tc (ºC) CO - 400ºC CO - 600ºC CO - 800ºC CO - 1000ºC CO - 1200ºC CO - 1400ºC Figure 8. Evolution of energy conversion efficiency as a function of carbonization temperature. Figure 9. (A) Relation between carbon conversion and time for carbonized olive stones, (B) Dependence of ignition, peak and burnout temperatures on peak pre-treatment temperature along with the burning time. 0.05 0.10 0.15 0.20 260 280 300 320 340 360 380 Ignition temperature, Ti (ºC) Volatile matter content fraction CO - 400ºC CO - 600ºC CO - 800ºC CO - 1000ºC CO - 1200ºC CO - 1400ºC Figure 10. Effects of volatile matter content on the ignition temperature of biochars. 18 5. References [1] Nishiguchi S, Tabata T. Assessment of social, economic, and environmental aspects of woody biomass energy utilization: Direct burning and wood pellets. Renewable and Sustainable Energy Reviews 2016;57:1279-86. [2] Ragauskas AJ, Williams CK, Davison BH, Britovsek G, Cairney J, Eckert CA, et al. The Path Forward for Biofuels and Biomaterials. Science 2006;311(5760):484-9. [3] Qian FP, Chyang CS, Huang KS, Tso J. Combustion and NO emission of high nitrogen content biomass in a pilotscale vortexing fluidized bed combustor. Bioresource Technology 2011;102(2):1892-8. [4] Eliche-Quesada D, Leite-Costa J. Use of bottom ash from olive pomace combustion in the production of ecofriendly fired clay bricks. Waste Management 2016;48:323-33. [5] Sanchez F, San Miguel G. Improved fuel properties of whole table olive stones via pyrolytic processing. Biomass Bioenerg 2016;92:1-11. [6] Roselló-Soto E, Koubaa M, Moubarik A, Lopes RP, Saraiva JA, Boussetta N, et al. Emerging opportunities for the effective valorization of wastes and by-products generated during olive oil production process: Non-conventional methods for the recovery of high-added value compounds. Trends in Food Science & Technology 2015;45(2):296- 310. [7] Martín-Lara MA, Ronda A, Blázquez G, Pérez A, Calero M. Pyrolysis kinetics of the lead-impregnated olive stone by non-isothermal thermogravimetry. Process Safety and Environmental Protection 2018;113:448-58. [8] Rodríguez G, Lama A, Rodríguez R, Jiménez A, Guillén R, Fernández-Bolaños J. Olive stone an attractive source of bioactive and valuable compounds. Bioresource Technology 2008;99(13):5261-9. [9] Petrou N, Budinova T, Razuigorova M, Parra J, Galiatsatou P. Conversion of olive wastes to volatiles and carbon adsorbents. Biomass Bioenerg 2008;32(12):1303-10. [10] Moreno N, Caballero Á, Morales J, Rodríguez-Castellón E. Improved performance of electrodes based on carbonized olive stones/S composites by impregnating with mesoporous TiO2 for advanced Li—S batteries. Journal of Power Sources 2016;313:21-9. [11] Garcia-Maraver A, Zamorano M, Ramos-Ridao A, Diaz LF. Analysis of olive grove residual biomass potential for electric and thermal energy generation in Andalusia (Spain). Renew Sust Energ Rev 2012;16(1):745-51. [12] Jiang L, Yuan X, Li H, Xiao Z, Liang J, Wang H, et al. Pyrolysis and combustion kinetics of sludge–camphor pellet thermal decomposition using thermogravimetric analysis. Energy Conversion and Management 2015;106:282-9. [13] Chacartegui R, Sanchez D, de Escalona JMM, Munoz A, Sanchez T. Gas and steam combined cycles for low calorific syngas fuels utilisation. Applied Energy 2013;101:81-92. [14] Gunasee SD, Carrier M, Gorgens JF, Mohee R. Pyrolysis and combustion of municipal solid wastes: Evaluation of synergistic effects using TGA-MS. Journal of Analytical and Applied Pyrolysis 2016;121:50-61. [15] Liu Z, Balasubramanian R. A comparison of thermal behaviors of raw biomass, pyrolytic biochar and their blends with lignite. Bioresource Technology 2013;146:371-8. [16] McKendry P. Energy production from biomass (part 1): overview of biomass. Bioresource Technology 2002;83(1):37-46. [17] Bell DA, Towler BF, Fan M. Coal Gasification and Its Applications. Elsevier Science; 2010. [18] Xiong S, Zhang S, Wu Q, Guo X, Dong A, Chen C. Investigation on cotton stalk and bamboo sawdust carbonization for barbecue charcoal preparation. Bioresource Technology 2014;152:86-92. [19] Demirbas A. Effects of temperature and particle size on bio-char yield from pyrolysis of agricultural residues. Journal of Analytical and Applied Pyrolysis 2004;72(2):243-8. [20] Sánchez F, Araus K, Domínguez MP, Miguel GS. Thermochemical Transformation of Residual Avocado Seeds: Torrefaction and Carbonization. Waste and Biomass Valorization 2017;8(7):2495-510. 19 [21] Mundike J, Collard F-X, Görgens JF. Pyrolysis of Lantana camara and Mimosa pigra: Influences of temperature, other process parameters and incondensable gas evolution on char yield and higher heating value. Bioresource Technology 2017;243:284-93. [22] Assis MR, Brancheriau L, Napoli A, Trugilho PF. Factors affecting the mechanics of carbonized wood: literature review. Wood Science and Technology 2016;50(3):519-36. [23] Kumar P, Kumar P, Rao PVC, Choudary NV, Sriganesh G. Saw dust pyrolysis: Effect of temperature and catalysts. Fuel 2017;199:339-45. [24] Lee J-W, Kim Y-H, Lee S-M, Lee H-W. Optimizing the torrefaction of mixed softwood by response surface methodology for biomass upgrading to high energy density. Bioresource Technology 2012;116:471-6. [25] Ronsse F, van Hecke S, Dickinson D, Prins W. Production and characterization of slow pyrolysis biochar: influence of feedstock type and pyrolysis conditions. GCB Bioenergy 2013;5(2):104-15. [26] Liao R, Gao B, Fang J. Invasive plants as feedstock for biochar and bioenergy production. Bioresource Technology 2013;140:439-42. [27] Lee Y, Eum P-R-B, Ryu C, Park Y-K, Jung J-H, Hyun S. Characteristics of biochar produced from slow pyrolysis of Geodae-Uksae 1. Bioresource Technology 2013;130:345-50. [28] Angın D. Effect of pyrolysis temperature and heating rate on biochar obtained from pyrolysis of safflower seed press cake. Bioresource Technology 2013;128:593-7. [29] He X, Liu Z, Niu W, Yang L, Zhou T, Qin D, et al. Effects of pyrolysis temperature on the physicochemical properties of gas and biochar obtained from pyrolysis of crop residues. Energy 2018;143:746-56. [30] Enders A, Hanley K, Whitman T, Joseph S, Lehmann J. Characterization of biochars to evaluate recalcitrance and agronomic performance. Bioresource Technology 2012;114:644-53. [31] Mata-Sánchez J, Pérez-Jiménez JA, Díaz-Villanueva MJ, Serrano A, Núñez-Sánchez N, López-Giménez FJ. Statistical evaluation of quality parameters of olive stone to predict its heating value. Fuel 2013;113:750-6. [32] Marcilla A, García AN, Pastor MV, León M, Sánchez AJ, Gómez DM. Thermal decomposition of the different particles size fractions of almond shells and olive stones. Thermal behaviour changes due to the milling processes. Thermochimica Acta 2013;564:24-33. [33] Morin M, Pécate S, Masi E, Hémati M. Kinetic study and modelling of char combustion in TGA in isothermal conditions. Fuel 2017;203:522-36. [34] Costa FF, Wang G, Costa M. Combustion kinetics and particle fragmentation of raw and torrified pine shells and olive stones in a drop tube furnace. Proceedings of the Combustion Institute 2015;35(3):3591-9. [35] Álvarez-Murillo A, Román S, Ledesma B, Sabio E. Study of variables in energy densification of olive stone by hydrothermal carbonization. Journal of Analytical and Applied Pyrolysis 2015;113:307-14. [36] Volpe M, Fiori L. From olive waste to solid biofuel through hydrothermal carbonisation: The role of temperature and solid load on secondary char formation and hydrochar energy properties. Journal of Analytical and Applied Pyrolysis 2017;124:63-72. [37] Missaoui A, Bostyn S, Belandria V, Cagnon B, Sarh B, Gökalp I. Hydrothermal carbonization of dried olive pomace: Energy potential and process performances. Journal of Analytical and Applied Pyrolysis 2017;128:281-90. [38] Guizani C, Haddad K, Jeguirim M, Colin B, Limousy L. Combustion characteristics and kinetics of torrefied olive pomace. Energy 2016;107:453-63. [39] Sanginés P, Domínguez MP, Sánchez F, Miguel GS. Slow pyrolysis of olive stones in a rotary kiln: Chemical and energy characterization of solid, gas, and condensable products. Journal of Renewable and Sustainable Energy 2015;7(4):043103. [40] García-Maraver A, Zamorano M, Ramos-Ridao A, Díaz LF. Analysis of olive grove residual biomass potential for electric and thermal energy generation in Andalusia (Spain). Renewable and Sustainable Energy Reviews 2012;16(1):745-51. [41] Garcia-Maraver A, Perez-Jimenez JA, Serrano-Bernardo F, Zamorano M. Determination and comparison of combustion kinetics parameters of agricultural biomass from olive trees. Renewable Energy 2015;83:897-904. 20 [42] Telmo C, Lousada J, Moreira N. Proximate analysis, backwards stepwise regression between gross calorific value, ultimate and chemical analysis of wood. Bioresource Technology 2010;101(11):3808-15. [43] McKendry P. Energy production from biomass (part 2): conversion technologies. Bioresource Technology 2002;83(1):47-54. [44] Byrne CE, Nagle DC. Carbonization of wood for advanced materials applications. Carbon 1997;35(2):259-66. [45] Zhang X, Xu M, Sun R, Sun L. Study on Biomass Pyrolysis Kinetics. Journal of Engineering for Gas Turbines and Power 2004;128(3):493-6. [46] Yang YB, Sharifi VN, Swithenbank J. Effect of air flow rate and fuel moisture on the burning behaviours of biomass and simulated municipal solid wastes in packed beds. Fuel 2004;83(11–12):1553-62. [47] Cagnon B, Py X, Guillot A, Stoeckli F, Chambat G. Contributions of hemicellulose, cellulose and lignin to the mass and the porous properties of chars and steam activated carbons from various lignocellulosic precursors. Bioresource Technology 2009;100(1):292-8. [48] Watkins D, Nuruddin M, Hosur M, Tcherbi-Narteh A, Jeelani S. Extraction and characterization of lignin from different biomass resources. Journal of Materials Research and Technology 2015;4(1):26-32. [49] Yeo JY, Chin BLF, Tan JK, Loh YS. Comparative studies on the pyrolysis of cellulose, hemicellulose, and lignin based on combined kinetics. Journal of the Energy Institute 2017. [50] Virmond E, De Sena RF, Albrecht W, Althoff CA, Moreira RFPM, José HJ. Characterisation of agroindustrial solid residues as biofuels and potential application in thermochemical processes. Waste Management 2012;32(10):1952-61. [51] Mansaray KG, Ghaly AE. Thermal degradation of rice husks in nitrogen atmosphere. Bioresource Technology 1998;65(1):13-20. [52] Jiang L-b, Yuan X-z, Li H, Chen X-h, Xiao Z-h, Liang J, et al. Co-pelletization of sewage sludge and biomass: Thermogravimetric analysis and ash deposits. Fuel Processing Technology 2016;145:109-15. [53] Khan AA, de Jong W, Jansens PJ, Spliethoff H. Biomass combustion in fluidized bed boilers: Potential problems and remedies. Fuel Processing Technology 2009;90(1):21-50. [54] García R, Pizarro C, Lavín AG, Bueno JL. Characterization of Spanish biomass wastes for energy use. Bioresource Technology 2012;103(1):249-58. [55] Nunes LJR, Matias JCO, Catalao JPS. Biomass combustion systems: A review on the physical and chemical properties of the ashes. Renew Sust Energ Rev 2016;53:235-42. [56] Jauhiainen J, Martin-Gullon I, Conesa JA, Font R. Emissions from pyrolysis and combustion of olive oil solid waste. Journal of Analytical and Applied Pyrolysis 2005;74(1–2):512-7. [57] Gutiérrez-Pardo A, Ramírez-Rico J, Arellano-López AR, Martínez-Fernández J. Characterization of porous graphitic monoliths from pyrolyzed wood. J Mater Sci 2014;49(22):7688-96. [58] Byrne CE, Nagle DC. Carbonized wood monoliths—Characterization. Carbon 1997;35(2):267-73. [59] Keybondorian E, Zanbouri H, Bemani A, Hamule T. Estimation of the higher heating value of biomass using proximate analysis. Energy Sources, Part A: Recovery, Utilization, and Environmental Effects 2017;39(20):2025- 30. [60] Qi H, Zhao B. Cleaner Combustion and Sustainable World. Springer Berlin Heidelberg; 2012. [61] Shen J, Fatehi P, Soleimani P, Ni Y. Recovery of lignocelluloses from pre-hydrolysis liquor in the lime kiln of kraftbased dissolving pulp production process by adsorption to lime mud. Bioresource Technology 2011;102(21):10035- 9. [62] Boumanchar I, Chhiti Y, M’hamdi Alaoui FE, El Ouinani A, Sahibed-Dine A, Bentiss F, et al. Effect of materials mixture on the higher heating value: Case of biomass, biochar and municipal solid waste. Waste Management 2017;61:78-86. [63] El-Sayed SA, Khairy M. Effect of heating rate on the chemical kinetics of different biomass pyrolysis materials. Biofuels 2015;6(3-4):157-70. [64] Shen Z, Liang Q, Xu J, Zhang B, Han D, Liu H. In situ experimental study on the combustion characteristics of captured chars on the molten slag surface. Combustion and Flame 2016;166:333-42. 21 [65] Lang T, Hurt RH. Char combustion reactivities for a suite of diverse solid fuels and char-forming organic model compounds. Proceedings of the Combustion Institute 2002;29(1):423-31. [66] Hurt RH. Structure, properties, and reactivity of solid fuels. Symposium (International) on Combustion 1998;27(2):2887-904. [67] McNamee P, Darvell LI, Jones JM, Williams A. The combustion characteristics of high-heating-rate chars from untreated and torrefied biomass fuels. Biomass and Bioenergy 2015;82:63-72.