HUNGARIAN JOURNAL OF INDUSTRY AND CHEMISTRY Vol. 50(2) pp. 27-33 (2022) hjic.mk.uni-pannon.hu DOI: 10.33927/hjic-2022-15 INVESTIGATION OF THE PYROLYSIS OF ANIMAL MANURE IN A LABORATORY-SCALE TUBULAR REACTOR: THE EFFECT OF THE PROCESS TEMPERATURE AND RESIDENCE TIME MARIA ELENA LOZANO FERNANDEZ1, SZABINA TOMASEK1*, CSABA FÁYKÖD2 AND ANDREA SOMOGYI3 1 Research Centre for Biochemical, Environmental and Chemical Engineering, Department of MOL Hydrocarbon and Coal Processing, University of Pannonia, Egyetem u. 10, Veszprém, 8200, HUNGARY 2 Felső-Bacska Storage Windpark Ltd., Arany János út 200, Fadd, 7133, HUNGARY 3 Solver Unio Ltd., Ábel Jenő u. 8, Budapest, 1113, HUNGARY This paper focuses on the pyrolysis of animal manure in a laboratory-scale tubular reactor between 300 and 900°C at nitrogen flow rates of 1 and 5 dm3/h. During the experiments, it was found that both the temperature and nitrogen flow rate had significant effects on the product yields and compositions. The highest gas yield and syngas content were observed at 900°C at a nitrogen flow rate of 1 dm3/h. In this case, since the gaseous product was characterized by a H2/CO ratio of 0:5, its quality must be improved prior to being used for synthesis. The composition of the solid residue was also affected by the pyrolysis parameters . Based on the hydrogen/carbon and oxygen/carbon ratios, it was concluded that both the water-gas shift and Boudouard reactions were the most critical. Keywords: cattle manure, pyrolysis, tubular reactor, product yield, composition 1. Introduction The rapid growth of the world’s population is leading to a significant rise in the demand and consumption of food, including meat and animal-derived products. As a consequence, farms and this sector generate huge amounts of waste such as livestock manure, sewage sludge and poultry litter [1]-[2]. The inadequate management of manure and sewage sludge causes serious health and environmental issues, e.g. water and air pollution as well as the emission of greenhouse gases and heavy metals in addition to the spread of pathogens. The composition of this residue includes a wide variety of chemical and biological compounds which are associated with the specific species of animals and age ranges amongst other factors. The residue consists of a complex mixture of compounds, mainly microbiota, lignocellulose, proteins as well as a significant amount of inorganic matter such as S, N, P, K, Ca, Mg and Cl. Additionally, heavy metals such as Cu, Pb, Cd, Zn and Mn can be found in the residue due to the use of antibiotics and hormones supplied to the animals [3]-[5]. Some of the practices concerning the disposal of sewage sludge and manure include landfilling, agricultural utilization, composting, anaerobic digestion and thermochemical conversion [6]. One of the most Received: 26 Sept 2022; Revised: 10 Oct 2022; Accepted: 11 Oct 2022 *Correspondence: tomasek.szabina@mk.uni-pannon.hu traditional and practical alternatives is the usage of these residues in agricultural land due to the high content of N and P which are essential elements required in plant fertilization. However, nowadays, this practice has diminished due to the enormous amount of waste generated, exceeding the nutritional requirements of the soil. Environmental regulations establish limits on the values allowed for the usage of sewage sludge in agriculture. The excessive use of manure on land causes problems such as contamination of the subsoil and surface, odors as well as the emission of greenhouse gases and ammonia [5,7-10]. Therefore, new alternatives for the proper management of this type of waste have been explored as well as studied more comprehensively in an attempt to solve the environmental and social impacts [8]. For example, thermal conversion techniques such as pyrolysis and gasification could be alternatives for transforming the residue into valuable products such as oil, char and gaseous products. Additional advantages of these techniques are that the huge volume generated is reduced, microorganisms are degraded and pathogenic organisms destroyed [1,9-10]. It is important to take into consideration that the percentage of humidity in the residues in the material is high and should be reduced while using the thermal techniques. The material could https://doi.org/10.33927/hjic-2022-15 mailto:tomasek.szabina@mk.uni-pannon.hu LOZANO FERNANDEZ, TOMASEK, FÁYKÖD AND SOMOGYI Hungarian Journal of Industry and Chemistry 28 be dried through natural, mechanical and thermal drying techniques [11]. Depending on the thermal degradation technique applied, the composition of the feedstock; process conditions, e.g. temperature and heating rate; as well as product distribution and its composition will vary [11]. For example, during pyrolysis, degradation in the absence of oxygen occurs at high temperatures. Through pyrolysis, the feedstock decomposes to form char, oil and light non-condensable gases [11]. In the case of the raw material containing a large amount of hemicellulose and cellulose, a product consisting of a higher percentage of gases could be expected, while lignin contributes towards the formation of char. Regarding the temperature, the formation of char is favored by low temperatures, while that of oil and volatile gaseous components is more likely at high temperatures [12]. The products can be further utilized depending on their characteristics. Char has interesting physical properties, for example, a high surface area, microporosity as well as high adsorption and ion exchange capacities. Char potentially could be used as an adsorbent, a catalyst, in power plants or as a fertilizer. Char obtained from manure is rich in elements such as K, P, Ca and Mg [10], [12]. In the oil fraction, hydrocarbons are produced but the pyrolysis oil from manure and sewage sludge is not regarded as of high quality since the oil produced contains oxygen, which gives rise to the production of compounds such as alcohols, ketones, aldehydes and esters that decrease the quality of the product [1]. The gas produced contains a mixture of CO, CO2, CH4, H2 and some light hydro- carbons (C2H2, C2H4, C2H6 and C3H8) [13]. The gas may be valuable because the energy it contains could be used in gas turbines and power plants or its compounds applied as a feedstock to be processed into a more added- value chemical product [13]-[14]. Despite the wide range of possible uses of the products, relatively few studies have investigated the pyrolysis of cattle manure. Consequently, limited information about the product yields and compositions is available. In light of the above, this study focuses on the pyrolysis of cattle manure within the temperature range of 300-900°C using nitrogen flow rates of 1 and 5 dm3/h as well as on the impact of the process parameters. 2. Materials and methods 2.1. Raw material Cattle manure was used as a feedstock for the pyrolysis experiments. Before the experiments, the raw material was dried at 110°C to constant mass. To determine the physical and chemical properties, the proximate and ultimate analyses of cattle manure (Table 1) were carried out. As data show, the raw material is characterized by 40.6% ash, 53.5% volatile compounds and 0.3% water content. The C, H, N, S and O contents were determined by a Carlo Erba-type elemental analyzer and in order to identify the inorganic compounds, e.g. Ca, P, S, Si, Na, Mg, Fe and Al, Energy Dispersive X-Ray Analysis was performed (Shimadzu EDX). 2.2. Pyrolysis experiments Pyrolysis was performed in a laboratory-scale horizontal tubular reactor (Fig.1). The cattle manure was placed in the center of a glass wool tube. The experiments were performed at 300, 500, 700 and 900°C. A N2 atmosphere was used and the flow rates established were 1 and 5 dm3/h. The heating rate used for this experiment was 100°C/min. The reaction system was equipped with a scrubber and silica gel-filled tube, where the gaseous products were purified to remove possible impurities and dried. At the end of the reaction, the product yields were estimated by measuring and calculating their difference in mass. 2.3. Product analysis The composition of the gaseous products was determined by a DANI-type gas chromatograph using a flame ionization and thermal conductivity detectors. The equipment contained two capillary columns (Rtx-1 PONA (100 m x 0.25 mm x 0.5 µm) and Carboxen TM-1006 PLOT (30 m x 0.53 mm)). Regarding the isothermal conditions of the PONA capillary column, the injector and detector temperatures were both 230°C. In terms of the Carboxen TM-1006 PLOT capillary column, the applied heating program was as follows: 35°C for 18 mins before being heated to 120°C at a heating rate of 15°C/min and maintained at 120°C for 2 mins. The Table 1. Proximate and ultimate analyses of cattle manure Parameter Value (%) P ro x im a te a n a ly si s Fixed carbon 5.59 Ash content 40.68 Volatile organic compounds 53.43 Water content 0.30 U lt im a te a n a ly si s C 24.80 H 3.00 N 2.90 S 1.30 O 45.30 Others (Ca, P, S, Si, Na, Mg, Fe, Al) 22.70 Figure 1. Experimental setup PYROLYSIS OF ANIMAL MANURE 50(2) pp. 27-33 (2022) 29 retention times of the components were determined using gas mixtures and individual components. 3. Results and Discussion 3.1. Product yields The product yields of the pyrolysis experiments are summarized in Fig.2. During the experiments, only gas and char were formed. As expected, the gas yields and amount of solid residue produced increased and decreased, respectively, as a function of the reaction temperature. During the pyrolysis process, a series of complex reactions took place. Up to 200°C, to all intents and purposes, only the water was removed from the cattle manure (Fig.3). In the torrefaction stage, since cellulose, hemicellulose and lignin were only slightly degraded, the final product was a solid carbonaceous material. This stage was followed by the pyrolysis process, where a significant reduction in mass resulted. In addition, a sharp peak appeared at approximately 300°C in the derivative thermogravimetric diagram (DTG), indicating that the reduction in mass occurred at a high speed. It is well known that cellulose degradation occurs between300 and 350°C, while protein decomposes between 450 and 660°C. In addition, deamination also took place. During the pyrolysis stage, approximately 30% of the initial mass was lost. Above 600°C, another significant proportion of mass, ~25%, was lost. This reduction in mass was related to the degradation of chains of lignin, carbon and minerals. It was also observed that the rate at which the mass reduced was low and relatively stable. This effect could be attributed to the minerals and possible carbonated forms. Additionally, it is worth mentioning that within this temperature range, CO2 was also formed. CO2 acts as an oxygen donor, promoting the Boudouard reaction and, therefore, the formation of CO. Another peak is also visible at 800°C in the DTG curve, which can be attributed to the devolatilization of the char and decomposition of the mineral matter. At the end of the test, that is, at a temperature of 900°C, the percentage of mass consisting of ash and fixed carbon remaining in the crucible was the same as that reported during the proximate analysis. It is important to note that the N2 flow rate also had an effect on the product yields. The lower flow rates facilitated the formation of the gaseous products. Given the longer residence time, the volatile vapors resuting from the pyrolysis exited the reactor more slowly, so they had sufficient time to degrade more comprehensively. 3.2. Composition of the gaseous products Although the gaseous products consisted of H2, CO, CO2 and CH4, C2-C6 hydrocarbons were also formed (Fig.4). H2 was formed by dehydrogenation, however, it could have formed as a result of the reforming reactions. CO may be related to the reactions that facilitated the cleavage of bonds in the ether groups and decarbonylation from proteins [15]-[16]. As Fig.4 shows, the formation of CO considerably increased above 700°C and the highest level was obtained at 900°C. This large increase was attributed to the Boudouard reaction, which has already been referred to in the thermogravimetric analysis. The Boudouard reaction can also be catalyzed by carbonates present in the manure [17]. The increase in CO can also be justified by a reaction between CO2 and other compounds generated during pyrolysis, the reduction of CO2 to CO Figure 3. Thermogravimetric results with regard to the raw material D ry in g T o rr e fa ct io n P y ro ly si s P y ro ly si s- g a si fi ca ti o n d m /d t W e ig h t lo ss , % 0. 0 1. 0 2. 0 3. 0 4. 0 5. 0 6. 0 0 25 50 75 10 0 Temperature, °C 200 400 600 800 1000 200 400 600 800 1000 Figure 2. Product yields of pyrolysis 0 20 40 60 80 100 300 500 700 900 Gas yield, % T e m p e r a tu r e , ° C 5dm3/h 1dm3/h 0 20 40 60 80 100 300 500 700 900 Char yield, % T e m p e r a tu r e , ° C 5dm3/h 1dm3/h LOZANO FERNANDEZ, TOMASEK, FÁYKÖD AND SOMOGYI Hungarian Journal of Industry and Chemistry 30 and simultaneously the oxidation of the carbon of the pyrolysis product through a homogeneous reaction [18]. The quality of the gaseous product can be defined in terms of its chemical composition and calorific value. Fig.5 represents the content of syngas, namely H2 and CO, in the product. From the results, it is remarkable that the production of syngas only took place above 500°C and the yield increased in proportion to the temperature. The highest syngas yields (~80%) were observed at 900°C. It is important to emphasize that the N2 flow rate also had an effect on the syngas yield. The yield of syngas was higher at lower N2 flow rates, in the same manner as the overall gas yield. The H2/CO ratio of syngas (Fig.6) is extremely important. The typical initial ratios for the transformation of methanol into chemicals are >2:1 for light olefins; <2:1 for diesel; 1.5:1 for aldehydes, higher alcohols and dimethyl ether; 1:1 for oxygen-containing alcohols and acetic acid; and 1:2 for polycarbonate [19]-[20]. As data in Fig.6 show, although the ratio of H2/CO at 500°C was approximately 1, at 700 and 900°C it was below 0.6. The heating values of the gases were also estimated. As is depicted in Fig.7, at lower temperatures (300 or 500°C) and an N2 flow rate of 1dm 3/h, the calorific value of the gas mixture was higher (~30 MJ/m3), meanwhile, at higher N2 flow rates, the heating value of the gaseous product was about 15 MJ/m3 at the same temperatures. This can be explained by the fact that a higher percentage of light hydrocarbons is present, the individual components of which provide a higher (N2 flow rate: 1 dm3/h) (N2 flow rate: 5 dm3/h) Figure 4. Composition of the gas products 0 20 40 60 80 300 500 700 900 Composition, % T e m p e r a tu r e , ° C C2-C6 CH4 CO2 CO H2 0 20 40 60 80 300 500 700 900 Composition, % T e m p e r a tu r e , ° C C2-C6 CH4 CO2 CO H2 (N2 flow rate: 1 dm3/h) (N2 flow rate: 5 dm3/h) Figure 5. Syngas yields 0 20 40 60 300 500 700 900 Yield, mmol/g raw material T e m p e r a tu r e , ° C CO, mmol/g H2, mmol/g 0 20 40 60 300 500 700 900 Yield, mmol/g raw material T e m p e r a tu r e , ° C CO, mmol/g H2, mmol/g Figure 6. H2/CO ratios of syngas 0,0 0,5 1,0 1,5 300 500 700 900 H2/CO ratio T e m p e r a tu r e , ° C 5dm3/h 1dm3/h Figure 7. Heating values of the gaseous products 0 10 20 30 40 300 500 700 900 Heating value, MJ/m3 T e m p e r a tu r e , ° C 5dm3/h 1dm3/h PYROLYSIS OF ANIMAL MANURE 50(2) pp. 27-33 (2022) 31 calorific value than the other compounds present in the mixture [21]. Another interesting correlation is the difference in the proportions of different elements contained in the gas as well as the resultant study and proposition of possible reaction mechanisms at different temperatures. The trends observed when the ratio of components were estimated are summarized in Table 2. From the process, the water-gas shift reaction and Boudouard reaction are some of the critical reactions that took place [14]. 3.3. Char According to the International Biochar Initiative (IBI), the char obtained from the pyrolysis experiments can be regarded as biochar [22], in which the C content is greater than 10% except for during the tests carried out at 900°C. To determine the possible applications of char, it is recommended to evaluate the relationship between the proportions of some elements. For example, the C/N ratio could be a positive parameter to determine the microbial activity should this residue be used in soils. Another aspect possibly worth evaluating and analyzing is the H/C and O/C ratios of the manure as well as the char obtained in each test (Table 3). The ratios are valuable for understanding the reaction mechanisms during pyrolysis under specific conditions [15]. The atomic ratios show that the process of carbonization changed the chemical compositions by removing functional groups. Due to the cleavage of the functional groups, the nitrogen contents significantly decreased, moreover, as a result of the formation of CO, CO2 and CH4, the carbon contents also reduced. Table 2. The proportions of different elements Component ratio N2 flow rate: 1 dm3/h N2 flow rate: 5 dm3/h Relationship Proposed reaction H2/CO Optimum (highest peak at 500°C) Optimum (highest peak at 500°C) Higher ratio at a flow rate of 5 dm3/h (except for at 500°C) 𝐶𝑛 𝐻𝑚 + 𝐻2 → 𝑛 + 𝑚 2𝐻2 + 𝑛𝐶𝑂 𝐶𝑂 + 𝐻2𝑂 → 𝐶𝑂2 + 𝐻2 CO/CO2 Increasing Increasing Higher ratio at a flow rate of 5 dm3/h 𝐶 + 𝐶𝑂2 → 2𝐶𝑂 H2/CH4 Optimum (highest peak at 500°C) Optimum (highest peak at 500°C) Higher ratio at a flow rate of 5 dm3/h 𝐶 + 𝐻2 → 𝐶𝐻4 CO/CH4 Optimum (highest peak at 700°C) Increasing Higher ratio at a flow rate of 5 dm3/h 𝐶𝑂 + 3𝐻2 → 𝐶𝐻4 + 𝐻2𝑂 CO2/CH4 Decreasing Decreasing Higher ratio at a flow rate of 5 dm3/h 𝐶𝑂2 + 4𝐻2 → 𝐶𝐻4 + 2𝐻2𝑂 2𝐶𝑂 + 2𝐻2 → 𝐶𝐻4 + 𝐶𝑂2 Table 3. Elemental analysis of the char products Cattle manure at N2 flow rate of 1 dm3/h at N2 flow rate of 5 dm3/h 300°C 500°C 700°C 900°C 300°C 500°C 700°C 900°C Elements content (%) C 24.8 23.40 20.20 20.90 6.30 23.50 20.50 18.90 7.20 H 3.00 1.80 0.70 0.30 0.80 1.60 0.60 0.30 0.80 N 2.90 2.50 1.50 1.00 0.40 2.30 1.60 0.80 0.80 S 1.20 1.50 1.40 1.40 2.50 1.40 1.60 1.40 2.90 O 45.30 48.20 53.50 53.70 67.30 48.50 53.10 55.80 65.60 Al 0.60 0.99 0.98 1.55 1.39 0.72 1.06 1.48 1.57 Ca 14.12 12.61 12.91 12.83 15.74 12.36 12.40 11.92 14.74 Cl 0.71 0.82 0.70 0.84 0.21 0.63 0.67 0.78 0.40 Fe 0.60 0.55 0.18 0.39 0.54 0.72 0.53 0.53 0.52 K 1.76 3.39 2.90 3.22 1.18 2.75 2.93 2.58 1.31 Mg 1.19 1.11 1.19 1.55 0.96 1.11 1.13 1.23 0.91 Mn 0.03 0.03 0.04 <0.001 <0.001 0.05 0.03 0.04 <0.001 P 2.16 1.40 1.99 2.32 0.96 1.93 2.09 2.05 1.17 Si 1.48 1.81 1.82 0.00 1.71 2.42 1.86 2.09 2.09 Atomic ratios C/N 8.6 9.4 13.5 20.9 15.8 10.2 12.8 23.6 9.0 H/C 0.10 0.08 0.03 0.01 0.13 0.07 0.03 0.02 0.11 O/C 1.8 2.1 2.6 2.6 10.7 2.1 2.6 2.9 9.1 LOZANO FERNANDEZ, TOMASEK, FÁYKÖD AND SOMOGYI Hungarian Journal of Industry and Chemistry 32 4. Conclusions In this study, cattle manure was pyrolysed in a horizontal tubular reactor between 300 and 900°C at N2 flow rates of 1 and 5 dm3/h. During the experiments, 20-60% gaseous and 40-80% solid carbonaceous residues were formed. The gas yields increased in proportion to the reaction temperature and residence time, while the amount of char decreased. The decomposition process resulted in the formation of H2, CO, CO2, CH4 and C2-C6 hydrocarbons. Syngas was only produced above 500°C. The ratio of H2/CO at 500°C was 1, while at 700 and 900°C, the proportion of CO was greater than that of H2. At lower temperatures (300 or 500°C) and at an N2 flow rate of 1dm3/h, the calorific value of the gas mixture was higher (~30 MJ/m3) than at the higher N2 flow rate and same temperatures (~15 MJ/m3). The process of carbonization changed the chemical composition of the raw material by removing functional groups, which also indicates the occurrence of the water-gas shift and Boudouard reactions. 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