Biochar Modification, Thermal Stability and Toxicity of Products Modification

Transactions of the VSB - Technical university of Ostrava Safety Engineering Series Vol. XII, No. 2, 2017 30 BIOCHAR MODIFICATION, THERMAL STABILITY AND TOXICITY OF PRODUCTS MODIFICATION Petra ROUPCOVÁ1, Romana FRIEDRICHOVÁ2, Karel KLOUDA3, Markéta WEISHEITELOVÁ4, Michaela PERĎOCHOVÁ5 1 VŠB - Technical University of Ostrava, Faculty of Safety Engineering, Ostrava, Czech Republic, [email protected] 2 Ministry of Interior - General Directorate of the Fire and Rescue Service of the Czech Republic, Prague, Czech Republic, [email protected].cz 3 VŠB - Technical University of Ostrava, Faculty of Safety Engineering, Ostrava, Czech Republic, [email protected] 4 State Offi ce for Nuclear, Chemical and Biological Protection, Kamenná, Czech Republic, [email protected] 5 VŠB - Technical University of Ostrava, Faculty of Safety Engineering, Ostrava, Czech Republic, [email protected] Abstract: Biochar is a product obtained from processing of waste biomass. The main application of biochar is in soil and environment remediation. Some new applications of this carbonaceous material take advantage of its adsorption capacity use it as a heterogeneous catalyst for energy storage and conversion etc. This contribution describes thermal stability of the original biochar. It discusses biochar modifi ed by chemical and physical methods including a new compound of biochar-graphene oxide. The purpose of the modifi cations is to increase its active surface to introduce active functional groups into the carbon structure of biochar in relation to fi re safety and toxicity of those products. Keywords: Biochar, soil remediation, thermal stability, fi re safety, ecotoxicity. Research article Introduction The raw material for preparation of biochar is biomass from diverse sources, e.g. cow and pig manure, straw, grass, corn, wood waste, sludge from wastewater treatment plants or a solid portion of digestate after biomass fermentation process. The basic technology for preparation of biochar is slow pyrolysis at 300-600 °C with limited or no access of air. Another variant is hydrothermal (wet) pyrolysis performed at lower temperatures, ca. 200 °C. Pyrolysis or hydrothermal pyrolysis produces a porous carbonaceous product with a more compact hydrophobic core with mostly aromatic structure, covered with a hydrophilic shell with some chemical activity associated with oxygen-containing functional groups. Biochar can be characterized with physical, chemical and hydraulic properties: • physical - size and distribution of particles, density, surface area, volume of pores (porosity); • chemical - red-ox potential, pH, content of O, H, C and their ratio, zeta potential, content of heavy metals, PAU content; • hydraulic - content of water leachate, humidity, hydraulic conductivity, absorbability. Use of biochar Biochar is a material with a number of potential applications but its properties can be infl uenced by many factors. They include the type of processed biomass, selection of technological procedure (pyrolysis, thermal pyrolysis), temperature, temperature gradient, reaction time etc. Its properties can be changed by the so-called modifi cation, e.g. increase of volume of pores, increase of specifi c surface, change of representation of functional groups on the surface or by introduction of nanoparticles of metals or metal oxides into the biochar structure. The modifi cation of biomass can be performed before pyrolysis, by addition of a catalyst (alkali hydroxides) or metal salts that are DOI 10.1515/tvsbses-2017-0012 Transactions of the VSB - Technical university of Ostrava Safety Engineering Series Vol. XII, No. 2, 2017 31 DOI 10.1515/tvsbses-2017-0012 reduced during pyrolysis to metal or metal oxide nanoparticles. Other methods include chemical and physical treatment of biochar after the pyrolysis. In most cases it involves chemical modifi cation seeking to change functional groups on the surface by oxidation, sulfonation, amidation or reaction of monomers (oligomers) to form composite materials. Physical modifi cations include sonication or turbo milling (one method for preparation of magnetic biochar). An overview of examples of published procedures for biochar modifi cation before and after pyrolysis is provided in Tab. 1. Tab. 1 Overview of published procedures for biochar modifi cation Agent used for modifi cation Modifi cation by pyrolysis Note to the content of the publication LO Before After CH3OH + Improvement of adsorption capacity to tetracycline (Jing et al., 2014) H2O2+ Utilization of the role of - OH radicals (G. Fang et al., 2014) FeCl2, FeSO4, Ni(NO3)2+ Preparation of magnetic biochar (Safarik et al., 2012) GRAPHITE+ Py-SO3+ Sorption material for PAU sorption (Zhang et al., 2012) KMnO4, HNO3+ Testing of changes of adsorption capacity (Li et al., 2014) KOH, HNO3, H2SO4, H2O2, KMnO4 +Changes of surface area, structure and volume of pores (Yakout et al., 2015) Sea weeds ? ? Electrochemical process (Jung et al., 2015) MgCl2+ Capture of phosphate ions (C. Fang et al., 2014) Active coal + For capture of PAU (Oleszczuk et al., 2012) H2O2, O3, KMnO4, HNO3+ Introduction of oxo groups on the surface (Liu et al., 2015) H2SO4+ Introduction of -SO3 group with surface catalysts (Liu et al., 2015) Nanoparticles of metals, Fe(NO3)3, Ni(NO3)2 +Introduction of metal nanoparticles into the structure (Liu et al., 2015) Monomers, oligomers, polymers + Preparation of polymer composite materials (Das et al., 2015) KOH, NaOH + Preparation of a catalyst of alkaline character (Abdul Hamid et al., 2014) HNO3, H2SO4+Improvement of adsorption capacity to Cd and Al ions (Qian et al., 2015) Metal salts (Fe, Pd, Pt, Ni) + Preparation of a catalyst biochar - metal (Shen, 2015) KOH, NaOH + Increase of biochar surface (Gu and Wang, 2013) Fe2+, Fe3+, SO4 2- + + Preparation of magnetic biochar (Tan et al., 2016) Clays, kaolin, montmorillonite + New material - testing of adsorption kinetics (methylene blue) (Yao et al., 2014) H2SO4, solution KOH + Improvement of adsorption capacity to tetracycline (P. Liu et al., 2012) HNO3 50 % + Formation of carbonaceous nanoparticles (Manav et al., 2016) CNT (0.01-1%) + Improvement of sorption capacity (Inyang et al., 2014) Lignite + Joint processing - hydrothermal (Z. Liu et al., 2012) MnCl2. 4H2O+ Modifi cation of biochar with MnOx Improvement of sorption capacity to As and Pb (Wang et al., 2015) KMnO4 + HCl + Preparation of composite Birnessite - Biochar (Wang et al., 2015) HNO3 + H2SO4 (1:1) + Na2S2O4+Amidation of biochar (reduction of NO2 groups to - NH2), increase of sorption capacity to Cu2+ (Yang and Jiang, 2014) Chitosan + Sorption of heavy metals and biological activity (Zhou et al., 2013) Fe3O4+Preparation of sorbents in a ball mill - sorption capacity to antibiotics (Shan et al., 2016) Legend: LO - Reference Transactions of the VSB - Technical university of Ostrava Safety Engineering Series Vol. XII, No. 2, 2017 32 worldwide-used pharmaceutical products, such as isobrufen (Essandoh et al., 2015), tetracycline (Liao et al., 2013), acetaminophenon (Im et al., 2014), aspirin (Essandoh et al., 2015) toxic industrial substances (Chen and Chen, 2009; Wang et al., 2013), explosives (Oh and Seo, 2014; Oh and Seo, 2015) etc. Potential negative factors of biochar Biochar toxicity Depending on the type of raw material (biomass) and the method of its pyrolysis, biochar can have a higher content of heavy metals, polyaromatic carbohydrates and various metabolites. During pyrolysis biochar can be contaminated with condensed pyrolytic vapors (Tang et al., 2016) - it is a process of the so-called release of mobile organic compounds. Electron paramagnetic resonance has shown that biochar also contains persistent free radicals that may cause inhibition of plant growth. As the main application of biochar is in soil it is subject to environmental monitoring, i.e. tests of phytotoxicity - germination, root elongation, plant growth, tests of escape behavior annelid worms, effects on soil microfl ora, behavior of biochar in soil organic matter etc. (Lehmann et al., 2011) Exposure to dust and fi re hazards Fire hazards are posed by the technology used for biomass processing and also by its products. The products are combustible and explosive gases (CH4, CO, H2, CnH2n+2), bio-oils (a liquid component) and biochar (a solid component). Biochar consists mainly of carbon-based powdery components which, depending on the surface area, size of particles, volume of pores and humidity, may form an explosive mixture with air. Safety data sheets of randomly selected three commercial biochar producers (Aemerge, Confl uence, U. S. Department of Labor) indicate the minimum explosive concentration of biochar 0.14 g/L. One work published in 2012 (Dzonzi-undi et al., 2014) dealt with determination of self-ignition parameters of biochar during storage in relation to the surrounding temperature and the biochar volume. At the surrounding temperature of 25 °C it is 5.3 m3 biochar on a heap while at 40 °C it is only 0.75 m3. The exposure limit in safety data sheets is 15 mg/m3 and for respirable particles it is 5 mg/m3. The Biochar Journal (Schmidt and Wilson, 2014) magazine published 55 examples of biochar utilization with potential commercialization. In addition to specifi c utilization opportunities (Yao and Wu, 2015; Zhang et al., 2014), such as heterogeneous catalyst with a broad fi eld of application, material for energy storage and conversion as a supercapacitator or Li-battery, the main application of biochar is in soil engineering and as an adsorbent for various inorganic and organic pollutants, both in water and soil environment - see below. In respect to agriculture (soil), biochar is recognized to have the following benefi ts (Krishnakumar et al., 2014): • increases water capacity of soil; • increases production of biomass; • increases pH of soil; • reduces toxicity of aluminum; • reduces tensile strength of soil; • changes (supports) microbial activity; • reduces emissions of CO2, N2O, CH4 from soil, adsorbs organic and inorganic compounds. The last reference is also connected with the important capacity of biochar to adsorb both inorganic substances (heavy metal ions) and organic contaminants in water environment and in soils, while it acts as a sorbent and prevents or limits undesired substances in plants. Physicochemical properties of biochar infl uence the adsorption kinetics. They are mainly the surface area, porosity and functional groups on the surface. Those properties are affected by the type of the initial biomass, temperature and duration of pyrolysis (hydropyrolysis), level of carbonization, pH of the solution in which the adsorption occurs, coexistence of cations, dosing (concentration) of biochar as an adsorbent, temperature (Note: adsorption is an endothermic process) etc. There is a number of mechanisms between the biochar surface and the adsorbed substance, such as electrostatic attraction, chemical bonds, intercalation, hydrogen bonds, π-π interaction, fi xing in pores, hydrophobic interaction etc. (Tan et al., 2015). Prevailing mechanisms have been demonstrated for adsorption of certain specifi c substances. For example, for trichloroethylene it was fi lling of pores (Ahmad et al., 2013) and for aromatic explosives (TNT) it was π-π interaction (Oh and Seo, 2014). There are also examples of adsorption of metal ions of chromium (Karim et al., 2015), lead (Lu et al., 2012), uranium (Kumar et al., 2011), zinc and copper (Chen et al., 2011) and a whole range of groups of organic substances which may have a negative impact on the environment. They are pesticides (Taha et al., 2014; Trigo et al., 2014; Sun et al., 2011), DOI 10.1515/tvsbses-2017-0012 Transactions of the VSB - Technical university of Ostrava Safety Engineering Series Vol. XII, No. 2, 2017 33 Sonication: NOTUS - Po wersonic s.r.o., Slovakia, Type PS 04000 A, 450 W. An overview of the chemical reactions and the physical treatments with identifi cation of products is shown in Fig. 1. We have used the Hummers method for joint oxidation of biochar and graphite in various weight ratios of graphite / biochar from 0 to 2. The total weight of the sample (biochar + graphite) was 2.25 g and the oxidizing mixture consisted of 52 ml of concentrated sulfuric acid, 3.0 g NaNO3 and after cooling of the mixture 10 °C 7.0 g of KMnO4 was gradually added. After that the reaction mixture was gradually heated to 55 °C and intensely stirred for 2-3 hours. Then it was left to stand at the laboratory temperature for 3-4 days. Subsequently the sample was diluted with distilled water, decanted and H2O2 and HCl were added. Centrifugation with decantation was repeated until pH was neutral and until a negative reaction to sulfate ions. Depending on its physical constitution the centrifuged product was either fi ltered or processed into a foil. In the case of FB-10 a part of the product was processed by lyophilization. Results and discussion We have divided this chapter into a part that compares structures and thermal stability of the initial biochars and a part that compares the foils prepared from the biochars by Hummers oxidation method while using the same initial ratio biochar - graphite (1:2). The third part contains a discussion of the effects of graphite share used in the Hummers oxidation method to get the fi nal product (powder, foil etc.) The last part contains results of some simple chemical and physical modifi cations. Introduction of rules for biochar standardization The European section of the International Biochar Initiative grants European Biochar Certifi cates to commercial producers (EBC www.Europen-biochar.org/en/ebc-ibi). To obtain the certifi cate it is necessary to submit and to document results of analyses of density, electric conductivity, pH, content of C, H, O, N, ash and heavy metals, PAU and TGA diagram. Data about thermal stability of biochar and its modifi cations published in this paper contribute to comprehensiveness of the doctoral research at FBI VŠB-TU in the fi elds of ecotoxicity and sorption capacity of biochar and its modifi cations. Material and methods Biochar: prepared by pyrolysis of biomass consisting of 80 % of corn silage and 20 % cellulose fi bers, 470 °C, 17 min. The pyrolysis was performed by Biouhel s.r.o Zlín. Biochar made of condensate (FB-1) and fl ue dust (FB-2). Instrumentation Measurement of FT-IR spectrums: Spectrometer Brucker Alpha/FT-IR, software OPUS 6.5., measuring range 375-4000 cm-1. TG-DTA and TG-DSC analysis: STA i 1500 (Instrument Specialists Incorporated - THASS), degradation medium: air, air fl ow rate 20 ml/min, temperature regime 25-600 °C, sample heating rate 10 °C/min, sample weights FB-1 9.70 mg, FB-2 9.31 mg, FB-3 9.41 mg, FB-4 10.50 mg, FB-5 10.13 mg, FB-6 2.62 mg, FB-7 9.64 mg, FB-8 1.76 mg, FB-9 1.89 mg, FB-10 4.06 mg. DOI 10.1515/tvsbses-2017-0012 Fig. 1 Biochar diagram Transactions of the VSB - Technical university of Ostrava Safety Engineering Series Vol. XII, No. 2, 2017 34 DOI 10.1515/tvsbses-2017-0012 Comparison of the initial biochars Comparison of foils prepared by joint oxidation of biochars (FB-1 and FB-2) and graphite (1 : 2) Fig. 2 FT-IR and thermal analyses of initial biochars FB-1and FB-2 [a) IRspectrum FB-1; b) TGA and DSC curves FB-1; c) IRspectrum FB-2; d) TGA and DSC curves FB-2] Fig. 3 FT-IR and thermal analysis of products of joint oxidation biochars FB-1 and FB-2 with graphite in the same ratio 1 : 2 [a) IR spectrum FB-6; b) TGA and DSC curves FB-6; c) IR spectrum FB-10; d) TGA and DSC curves FB-10] Transactions of the VSB - Technical university of Ostrava Safety Engineering Series Vol. XII, No. 2, 2017 35 DOI 10.1515/tvsbses-2017-0012 Effects of graphite quantity on the character of the product of joint oxidation of graphite and FB-2 using the Hummers method Fig. 4 FT-IR and thermal analysis of the product of oxidation of biochar FB-2 using the Hummers method without graphite: a) IR spectrum FB-7; b) TGA and DSC curves FB-7 Fig. 5 FT - IR and thermal analysis of products of joint of oxidation of biochar FB - 2 and graphite in different weight ratios [a)IR - spectrum FB-6; b) TGA and DSC curves FB-6; c) IR - spectrum FB-8; d) TGA and DSC curves FB-8; e) IRspectrum FB-9; f) TGA and DSC curves FB-9] Transactions of the VSB - Technical university of Ostrava Safety Engineering Series Vol. XII, No. 2, 2017 36 DOI 10.1515/tvsbses-2017-0012 Fig. 6 Summary comparison of IR spectrums and TGA and DSC curves for products of joint oxidation of biochar (FB-2) with graphite in different weight ratios (FB-6, FB-8, FB-9) Simple chemical modifi cation (with hydrogen peroxide, ascorbic acid) and sonication of biochar FB-2 Fig. 7 Thermal analysis (TGA and DSC) of the product FB-3 after physical treatment (sonication) of biochar FB-2 Fig. 8 Thermal analysis (TGA and DSC) of the product after chemical treatment of biochar FB-2: a) TGA and DSC curves of the product FB-4 (oxidation); b) TGA and DSC curves of the product FB-5 (ascorbic acid) Transactions of the VSB - Technical university of Ostrava Safety Engineering Series Vol. XII, No. 2, 2017 37 interval 428.4-599.6 °C there is an indication of an unfi nished exothermic process accompanied by a signifi cant weight loss of the sample by 33.6 wt. % in the measured interval of temperatures. On a DSC curve of the sample FB-4 one peak was detected corresponding to an endothermic process and two peaks corresponding to exothermic processes. The indistinct endothermic process is probably associated with a delay in heat transfer to the sample in the temperature interval 13.8-111.5 °C with the maximum at 44.4 °C and the peak area under the DSC curve is 247.6 kJ/kg. The fi rst exothermic process is also indistinct and it occurs in the temperature interval 307.7-394.0 °C with the peak area under the DSC curve -119.5 kJ/kg with the maximum at 356.6 °C. This process is continually followed by an unfi nished exothermic process in the temperature interval 394.0-599.7 °C and it is accompanied by a signifi cant weight loss of the sample by 35.5 wt. % in the interval of measured temperatures. On a DSC curve of the sample FB-5 one peak was detected corresponding to an endothermic process and two peaks corresponding to exothermic processes. The indistinct endothermic process occurs in the temperature interval 15.8-77.5 °C with the maximum at 37.0 °C and with the peak area under the DSC curve 80.9 kJ/kg. The fi rst exothermic process, also indistinct, occurs in the temperature interval 300.3-382.6 °C with the peak area under the DSC curve 118.3 kJ/kg and with the maximum at 346.8 °C. This process is continually followed by an unfi nished exothermic process in the temperature interval 397.1-599.8 °C and it is accompanied by a signifi cant weight loss of the sample by 36.8 wt. % in the interval of measured temperatures. The shapes of curves of the samples No. FB-3, FB-4 and FB-5 are very similar but in the case of the sample FB-3 the indistinct exothermic process is missing in the temperature interval ca. 300-390 °C. Samples FB-3 and FB-5 demonstrated a slight increase of weight at temperatures up to ca. 35 °C. On a DSC curve of the sample FB-7 one peak was detected corresponding to an endothermic process and one distinct exothermic process. The endothermic process occurs in the temperature interval 12.2-148.9 °C with the maximum at 90.7 °C and the peak area under the DSC curve is 712.8 kJ/kg. The distinct exothermic process is probably composed of two processes as it is obvious that it was not completed within the interval of measured temperatures. It occurs in the temperature interval 375.5-599.9 °C with the maximum at 576.2 °C and the peak area under the DSC curve Interpretation of the measured TGA and DSC curves - thermal stability of the prepared products The TGA curve of the samples No. 1 through 10 can be divided into several sections with different slopes, i.e. rates of weight loss. This division with the respective temperature intervals and weight losses contains parameters of detected thermal processes on the DSC curve. The energy change of the thermal process (ΔH) was determined as an area under a peak corresponding to the respective thermal process, i.e. as the area outlined by a DSC curve and a line between points indicating the beginning and end of the thermal process. The area under the peak is directly proportionate to the heat released or consumed in the reaction and the height of peak (Hf1) is directly proportionate to the reaction rate. On a DSC curve of the sample FB-1 one peak was detected representing an endothermic process and one peak corresponding to a combination of exothermic processes. The endothermic process occurs in the temperature interval 13.1-129.3 °C with the minimum at 84.0 °C and with the peak area under the DSC curve 1258.6 kJ/kg and it is accompanied by a big weight loss by 25.4 wt. %. The exothermic process in the temperature interval 242.5-599.7 °C with the peak area under the DSC curve 6655.5 kJ/kg and with maximum at 416.1 °C is made up of several processes that cannot be differentiated. Also in this case there is a signifi cant weight loss of the sample. On a DSC curve of the sample FB-2 one peak was detected corresponding to an endothermic process and one peak corresponding to more exothermic processes. An indistinct endothermic process, associated probably with a delay in heat transfer to the sample, occurs in the temperature interval 13.1-81.5 °C with the minimum at 23.0 °C and the peak area under the DSC curve is 124.3 kJ/kg. The exothermic process is probably made up of several, mutually overlapping thermal processes in the temperature interval 315.2-579.1 °C with the peak area under the DSC curve -7813.6 kJ/kg and with the maximum 554.1 °C. This thermal process is accompanied by a signifi cant weight loss by 50.9 wt. %. On a DSC curve of the sample FB-3 one peak was detected corresponding to an endothermic process and an indication of an exothermic process. In the temperature interval 16.2-42.0 °C there is an endothermic process with the peak area under the DSC curve 72.9 kJ/kg and it is probably associated with heat transfer to the sample. This process has the minimum at 24.0 °C. In the temperature DOI 10.1515/tvsbses-2017-0012 Transactions of the VSB - Technical university of Ostrava Safety Engineering Series Vol. XII, No. 2, 2017 38 the minimum at 72.5 °C. The fi rst exothermic process occurs in the temperature interval 192.3-254.6 °C with the maximum at 226.7 °C and with the peak area under the DSC curve at -865,5 kJ/kg. This process is accompanied by a signifi cant weight loss. The second exothermic process occurs in the temperature interval 450.9-559.5 °C with the peak area under the DSC curve -3762.2 kJ/kg and with the maximum at 530.3 °C. Also this process is accompanied by a signifi cant weight loss with a slight weight increase at the beginning. The foils FB-6, FB-8, FB-9 and FB-10 prepared by joint oxidation of biochar with graphite are products with the same functional groups (see Tab. 4 and spectrums in Fig. 6 and 3c)). This is refl ected by their very similar behavior during thermal analysis - see the TGA and DSC curves in Fig. 6, Fig. 4 and Fig. 3d. In all cases there are two exothermic processes with the maximums in the interval 212-230 °C. Slightly greater is the variation of temperatures of the second exothermal process where the maximums range from 488 to 530 °C. The lowest maximum temperature of degradation was found for the foil FB-9 which was made by oxidation with the lowest content of graphite. The total thermal changes of degradation of the foils FB-6, FB-8 and FB-9 are comparable but they are signifi cantly lower than that of biochar FB-2 alone, which in this case was the initial component. Different types of biochar affected the resulting total thermal change of degradation of foils with the same ratio of graphite (1:2): FB-6 (∑ H = - 4721 kJ/kg) in comparison with FB-10 (∑ H = -6760 kJ/kg), which does not correspond to the values identifi ed for thermal degradation of the initial biochars (sediment, fl ue ash). Biochar FB-2 was more exothermic (by 40 %) than FB-1 and the difference was 2294 kJ/kg. If we compare the total thermal change during degradation of foils prepared earlier (Klouda et al., 2014b) by oxidation of graphite alone and graphite with fullerene C60 (ratio 2:1) then we can see that the thermal change in case of the foil with biochar FB-2 (also the ratio 2:1) is higher and that the maximum temperature of the second exothermic effect is shifted to higher values, by up to 70-110 °C (depending on the graphite ratio). Oxidation with H2O2, slight reduction with ascorbic acid and sonication resulted in thermal stabilization of biochar. We assume that “natural organic matter” adsorbed on the carbon skeleton of biochar was removed by chemical and physical reactions. We tested products of modifi ed biochar in the temperature range 25-600 °C and the lowest weight loss was found for FB-3, i.e. for biochar is - 2418.3 kJ/kg. This process is accompanied by a signifi cant weight loss of the sample by ca. 62 wt. %. A visual inspection of the sample residues after the thermal degradation has shown that residues of the samples FB-1, FB-2 and FB-7 were in the form of brown and white fl y ash. The look of the samples FB-3, FB-4 and FB-5 remained practically unchanged, only on the sample surface there was a thin layer of light-colored fl y ash. On a DSC curve of the sample FB-6 one peak was detected corresponding to an endothermic process and two peaks corresponding to exothermic processes. The endothermic process is indistinct and it occurs in the temperature interval 15.1-47.9 °C with the peak area under the DSC curve 286.2 kJ/kg and with the minimum at 30.7 °C. The fi rst exothermic process occurs in the temperature interval 177.3-263.0 °C with the peak area under the DSC curve -1005.6 kJ/kg and with the maximum at 212.2 °C. The second exothermic process occurs in the temperature interval 458.3-591.8 °C with the peak area under the DSC curve -4001,8 kJ/kg and with the maximum at 519.9 °C. Both the exothermic processes are accompanied by signifi cant weight losses. On a DSC curve of the sample FB-8 two peaks were detected corresponding to exothermic processes. The fi rst exothermic process occurs in the temperature interval 179.7-246.2 °C with the maximum at 217.1 °C and with the peak area under the DSC curve 1319,2 kJ/kg. The second exothermic process occurs in the temperature interval 453.9-564.7 °C with the peak area under the DSC curve -4273,3 kJ/kg and with the maximum at 524.2 °C. Both the thermal processes are accompanied by signifi cant weight losses. On a DSC curve of the sample FB-9 two peaks were detected corresponding to exothermic processes. The fi rst exothermic process occurs in the temperature interval 183.3-255.9 °C with the maximum at 230.2 °C and with the peak area under the DSC curve -1033,3 kJ/kg. The second exothermic process occurs in the temperature interval 423,2-546,8 °C with the peak area under the DSC curve -3640,7 kJ/kg and with the maximum at 488.2 °C. Both the thermal processes are accompanied by signifi cant weight losses. On a DSC curve of the sample FB-10 one peak was detected corresponding to an endothermic process and two peaks corresponding to exothermic processes. The endothermic process occurs in the temperature interval 13.6-178.1 °C with the peak area under the DSC curve 1953,1 kJ/kg, which is probably associated with a delay in heat transfer to the sample. The process reaches DOI 10.1515/tvsbses-2017-0012