Thermal Decomposition and Combustion of Peat Fuel

ISSN 0361-5219, Solid Fuel Chemistry, 2019, Vol. 53, No. 5, pp. 283–288. © Allerton Press, Inc., 2019. Russian Text © The Author(s), 2019, published in Khimiya Tverdogo Topliva, 2019, No. 5, pp. 33–38. Thermal Decomposition and Combustion of Peat Fuel P. A. Maryandysheva,*, I. I. Mukhrevaa,**, V. K. Lyubova,***, C. Schonnenbeckb,****, G. Trouve b,*****, A. Brillardb,******, and J.-F. Brilhacb,******* a b Laboratoire Northern (Arctic) Federal University, Arkhangelsk, 163000 Russia Gestion des Riques et Environnement, Institut de Rescherche Jean-Baptiste Donnet, Universite de Haute-Alsace, Mulhouse, 68093 France *e-mail: [email protected] **e-mail: [email protected] ***e-mail: [email protected] ****e-mail: [email protected] *****e-mail: [email protected] ******e-mail: [email protected] *******e-mail: [email protected] Received February 7, 2019; revised February 20, 2019; accepted June 3, 2019 Abstract—The thermal decomposition of peat in a drop tube furnace was studied under conditions typical of the low-temperature combustion chambers of industrial heat-generating plants, and the combustion of peat charcoal was examined on a thermogravimetric analyzer with dynamic heating. It was established that the thermal properties of peat are well combined with the properties of woody biomass, primarily, with coniferous wood (spruce and pine). The main differences are that the process of peat pyrolysis occurred in a wider temperature range and two extremum points at temperatures of 277 and 320°C were observed in this case. Keywords: peat fuel, thermal decomposition, combustion, combustion chamber, pyrolysis, renewable energy source, thermogravimetric analysis, drop tube furnace, peat charcoal DOI: 10.3103/S0361521919050094 INTRODUCTION The energy released in the process of burning solid fuels, such as coal and wood biomass, is mainly used for the production of heat and electrical energy. The widespread use of coal and wood fuel can be explained by their relatively low cost and availability. Various coals from lignite (the youngest low-grade brown coal) to anthracite are used in many heat-generating installations. The combustion of coal fuels has received much attention, and it has been considered in sufficient detail in many publications including those concerning the use of numerical simulations [1–5]. Plants that use biomass as a fuel have been also considered in the technical literature, and a large number of works were devoted to the studies of processes occurring in fluidized-bed furnaces and in combustion chambers equipped with reciprocating grates. In this case, the formation of harmful gaseous and solid ingredients, including carbon black, was considered along with the problems of combustion, heat transfer, aerodynamics, and heating surface pollution [6–8]. To simulate the processes occurring with fuel in the combustion chamber of a heat-generating plant, a drop tube furnace is used. The design of this laboratory unit makes it possible to ensure a high temperature level and the composition of a gas atmosphere characteristic of the furnace chambers of industrial heat-generating plants [9–13]. The issues of simulating the combustion of coal fuel in a high-speed heating reactor were considered in detail [14, 15]. The results of studying the kinetics of coal gasification, the formation of soot particles, the formation of ash, and the pyrolysis of coal fuel in various atmospheres were reported [16, 17]. To decrease the anthropogenic impact of heatgenerating plants on the environment, it is necessary to develop new technologies for the combustion of coal fuel and fuel compositions with carbon neutral solid fuels, which do not cause greenhouse gas emissions, as the main combustible components. Peat is formed at an intermediate stage of the process of mineralization from biomass to coal fuel; it occupies an intermediate place between renewable and non-renewable energy sources and belongs to slowly renewable fuels because its recovery period at the place of production exceeds 200 years. However, the consumption of peat in the Russian Federation is 283 284 MARYANDYSHEV et al. Table 1. Thermotechnical characteristics of peat from the Mezen deposit on an analytical basis Sample Volatile content, % Peat 68.24 Bound carbon, % Moisture content, % 21.02 far behind its annual natural growth, which makes it possible to consider peat as a renewable source of energy in these conditions. High-moor, transitional, and lowland peat deposits can be used as fuel. Peat has many economic and environmental advantages, such as low sulfur and ash contents and minimal mercury content, and its lower calorific value on a combustible basis is close to that of brown coal. It should be noted that peat is a cheaper type of fuel than fuel oil and natural gas, and its price is comparable to that of wood biofuels. Peat deposits, which are widespread in almost the entire territory of the Russian Federation, play a significant role among fossil fuels. According to Afanas’ev et al. [18], the largest amount of peat reserves of A + B + C1 categories, 6.9 billion tons (36.2% of the reserves of the Russian Federation), has been explored in the North-West Federal District with a 5–10% degree of peat formation. The total number of peat deposits in this district is 18 912. Of all of the regions in the North-West Federal District, Arkhangelsk oblast has the largest peat reserves of 1.1 billion tons at a 40% moisture content. In addition, 23.7 billion tons were assigned to the rank of forecast resources, which will be promising for further development upon confirming by relevant exploration. The discovery of all peat deposits and manifestations in the Arkhangelsk oblast is directly related to the period of the mainstream use of peat as a fuel resource. Peat resources are recognized as a unique natural potential of organic origin, which affects the improvement of living standards. This is an energy-generating, industrial, and agrochemical resource necessary for both the development of power engineering and industry and an increase in the productivity of agriculture. With the development of science, it becomes a reliable source for biotechnology, health care, etc. In the modern literature, there are works devoted to studies of peat combustion processes; however, they mainly considered the natural smoldering of peat [19– 22] or the results of the combined combustion of peat and wood fuels [23] in various combustion devices and combustion methods [24]. The application of peat as a fuel in power generation is an effective way of using peat. The purpose of this work was to study the thermal decomposition of peat in a drop tube furnace under conditions typical of the low-temperature combustion chambers of industrial heat-generating plants and the subsequent combustion of peat charcoal in a thermogravimetric analyzer with dynamic heating. 10.74 Ash content, % Low heat value, MJ/kg 2.75 15.47 EXPERIMENTAL Thermogravimetric analysis. The samples of peat were taken in the Mezen district of the Arkhangelsk oblast. This district has very large peat reserves (more than 500 million tons); nevertheless, expensive imported coal dominates in the fuel and energy balance of this district, and its fraction is higher than 96%. To replace the expensive coal, it is necessary to carry out a set of studies for the peat deposits of the Mezen district, including heat-engineering studies and thermogravimetric (TG) analysis. The TG analysis was performed on a TA Instruments Q500 analyzer. The test peat samples of about 10 mg were placed in aluminum crucibles and heated to 700°C at a constant heating rate of 5 K/min in air and inert gas atmospheres. The samples of peat charcoal collected in the course of studying the pyrolysis of peat particles in a drop tube furnace were studied only in an atmosphere of air. The TG and differential thermogravimetric (DTG) curves were recorded during the entire process of heating. The moisture content, ash content, and volatile content of the test peat samples and the resulting peat charcoal were determined by standard procedures. The results obtained using standard procedures (Table 1) were compared with the thermogravimetric analysis data. An IKA C 2000 Basic Version 2 calorimeter was used to determine the specific heats of combustion of the test samples. Drop tube furnace. Figure 1 shows a schematic diagram of the drop tube furnace, which was developed in the Laboratory of Risk Management and Environment (University of Upper Alsace). This reactor consists of the following three main parts: a fuel supply system, a reaction zone, and a system for the collection of thermal decomposition and combustion products. The reaction zone was made of an aluminum– silicon tube with an inner diameter of 70 and an outer diameter of 80 mm. It was heated with a Nabertherm RHTV 80/1000/17s electric furnace and consisted of five heating zones with a total length of 1 m. The maximum power of the furnace and the heating temperature were 17.1 kW and 1600°C, respectively. The experiments with peat were carried out at a temperature of 800°C and different residence times in the reactor. The residence time was regulated by changing the height of the reaction zone of the reactor. The particle size of peat introduced into the reactor was 0.20–0.25 mm. The procedure used for calculating the rate of heating of fuel particles was described in detail by Zellagui et al. [15]; in these studies, this rate ranged from 10 4 to 105 K/s. SOLID FUEL CHEMISTRY Vol. 53 No. 5 2019 THERMAL DECOMPOSITION AND COMBUSTION OF PEAT FUEL Fuel supply system 285 Flow meter O2 N2 CO2 Air Primary gas Secondary gas Charging spout for fuel Electric furnace Reactor Reaction zone Trap pipe Sampler Fig. 1. High-speed heating reactor. TG, % 100 90 80 70 60 50 40 30 20 10 0 Initial sample (a) 1 2 100 200 300 400 500 600 700 Temperature, °С DTG, % 0.12 0.10 0.08 0.06 0.04 0.02 0 Initial sample (b) 1 2 100 200 300 400 500 600 700 Temperature, °С Fig. 2. (a) TG and (b) DTG curves of peat fuel: (1) TG curve in air and (2) TG curve in an inert atmosphere. The fuel supply system, which consisted of a syringe, a pump, and a rotating brush, was located above the upper part of the furnace to ensure the uniform supply of finely ground fuel into the spoon with a mass flow rate from 1 to 20 g/h. A 6-g sample of peat was placed in the syringe. The internal diameter of the spout was 8 mm, and the outer diameter was 18 mm. To exclude thermal effects on the peat particles before they enter the reaction chamber, the spout was equipped with a water cooling system. The size of the reaction zone through which particles fly when they fall is determined by a distance from the outlet section of the spout to the inlet section of the trap pipe of the sampler located in the lower part of the reaction chamber. The height of the trap pipe was adjustable; the trap pipe had an inner diameter of 16 mm and an outer diameter of 30 mm, and it was equipped with SOLID FUEL CHEMISTRY Vol. 53 No. 5 2019 a water cooling system. In the course of experiments, the height of the reaction zone was changed over a range from 17 to 40 cm at a heating temperature of 800°C. This temperature was chosen based on the following two considerations: – it provides an almost complete yield of volatile substances in a relatively short period of time and the possibility of a detailed description of this process, and – it is typical for many zones of industrial installations in which low-temperature fuel combustion technologies are implemented. In the course of an experiment, charcoal formed after the pyrolysis of peat fuel entered into a collector of solid reaction products, which was located in the lower part of the trap pipe. The rapid cooling of the products entering the sampler makes it possible to 286 MARYANDYSHEV et al. Yield of volatile substances, % (on a dry ash-free basis) 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 100 150 200 250 300 350 400 450 500 Time, ms Fig. 3. Yield of volatile substances from peat particles as a function of residence time in the high-speed heating reactor. avoid the subsequent reactions. The vertical position of the sampler trap pipe simplifies changes in the height of the reaction chamber and, accordingly, the residence time of fuel particles in the high-temperature zone with their uniform distribution over the reactor cross section. The collected samples of peat charcoal were studied in the thermal analyzer. RESULTS AND DISCUSSION Slow pyrolysis and combustion of peat fuel. TG and DTG analysis is often used to study the processes of slow pyrolysis and combustion under dynamic heating to simulate the temperature level and weight loss with a high accuracy. Figure 2 shows the TG and DTG curves for the initial peat samples. The pyrolysis and burning of peat fuel on dynamic heating at a rate of 5 K/min early came into play and completed in a temperature range of 500–570°C (Fig. 2). Because peat mainly consists of organic components (hemicellulose, cellulose and lignin), its thermal properties are well combined with the properties of woody biomass, primarily, with spruce and pine softwood. The main difference is that the peat pyrolysis process takes place in a wider temperature range with two extremum points observed at temperatures of 277 and 320°C (Fig. 2). In addition, the ash content of peat is higher than that of wood and its bark. Two distinct peaks (in addition to the peak of drying) are observed in the DTG curve of peat burning. The first peak with a maximum at a temperature of 293°C characterizes the release and combustion of volatile substances, and the second with a maximum at a temperature of 405°C corresponds to the combustion process of a carbon fuel matrix. The DTG curve of the wood burning process also has two characteristic peaks, but the first one is significantly higher than the second one [25]. The shape of the DTG curve of spruce bark is closest to that of peat. Popova et al. [25] reported a detailed comparative analysis of the TG and DTG studies of wood fuels and coals from different fields; the experimental results of the thermogravimetric analysis of peat with a low degree of decomposition confirmed the intermediate position of peat between wood–plant residues and brown coal. Isothermal process of peat pyrolysis in a high-speed heating reactor. In the experiments conducted in the high-speed heating reactor, the temperature was 800°C. Figure 3 shows the yield of volatile substances as a function of the residence time of peat particles in a reaction chamber filled with nitrogen. Mathematical representations obtained by Zellagui et al. [12] were used in the determination of the residence time of peat particles in the reaction chamber depending on its height. Thus, the abscissa axis in Fig. 3 characterizes the residence time of peat particles in the reaction zone when its height changed from 17 to 40 cm. The experiments showed that the yield of volatile substances in an atmosphere of nitrogen increased with the residence time of peat particles in the reaction zone of the installation. For a given residence time of peat particles in the reactor, the yield of volatile substances increased with temperature. The almost complete release of volatile substances from peat fuel with an initial particle size of 0.20–0.25 mm was achieved at a residence time of 450 ms in the reaction chamber. This value was achieved when the height of the reaction chamber was 40 cm. The maximum yield of volatile substances was about 79% (on a dry ash-free basis) for peat from the Mezen deposit. TG and DTG analysis of charcoal samples obtained from peat. The charcoal of peat fuel collected in a highspeed heating reactor at different peat residence times in the reaction zone with a temperature of 800°C was anaSOLID FUEL CHEMISTRY Vol. 53 No. 5 2019 THERMAL DECOMPOSITION AND COMBUSTION OF PEAT FUEL DTG, % 0.12 0.10 0.08 0.06 0.04 0.02 0 40 (a) 1 2 3 4 140 240 340 440 540 640 Temperature, °С TG, % 100 90 80 70 60 50 40 30 20 10 0 40 140 287 (b) 1 2 3 4 240 340 440 540 640 Temperature, °С Fig. 4. (a) DTG and (b) TG of the thermal degradation under air of peat charcoal obtained in the isothermal peat pyrolysis process at 800°C, exposure times: (1) 150 ms, (2) 250 ms, (3) 330 ms, and (4) 450 ms. lyzed in the thermogravimetric analyzer in an atmosphere of air. Figure 4 shows the TG and DTG curves. We experimentally found that the charcoal of peat obtained upon a longer thermal treatment ignited at higher temperatures; in this case, the height of the first peak characteristic of the ignition and combustion process of volatiles decreased with the duration of heat treatment, and this peak was almost absent at an exposure time of 450 ms (Fig. 4a). An opposite behavior was observed for the second peak characteristic of the ignition and combustion processes of the coke residue. The height of this peak increased with the duration of the heat treatment of peat particles in an inert atmosphere (Fig. 4a); of course, the ash content of the tested peat charcoal increased (Fig. 4b). Note that the temperature and time intervals of the combustion of peat charcoal increased, as compared with those of the original peat. The temperatures characteristic of the first peak maximum (Fig. 4a) increased with the time of the heat treatment of peat, and those of the second peak decreased. This fact indicates that the reaction properties of peat charcoal increased with the duration of the high-temperature processing of peat in an inert atmosphere to increase its burnout rate. The lower the volatile content of the peat charcoal, the greater the similarity of DTG curves obtained on the combustion of charcoals and coals. CONCLUSIONS The process of pyrolysis and combustion of peat fuel with dynamic heating at a rate of 5 K/min begins at 200–220°C and ends in a temperature range of 500–570°C. Because peat mainly consists of organic components, its thermal properties are well combined with the properties of woody biomass, primarily, with softwood (spruce and pine). The main difference is that the process of peat pyrolysis takes place in a wider temperature range, and two extremum points at temperatures of 277 and 320°C are observed in this case. Two distinct peaks are observed in the DTG curve of peat burning. The first peak with a maximum at a temSOLID FUEL CHEMISTRY Vol. 53 No. 5 2019 perature of 293°C characterizes the release and combustion of volatile substances, and the second with a maximum at a temperature of 405°C corresponds to the combustion process of the carbon matrix of the fuel. The DTG curve of wood burning also exhibited two peaks, and the first peak was significantly higher than the second. The shape of the DTG curve of spruce bark is closest to that of peat. The results of the thermogravimetric analysis of peat confirm its intermediate position between wood–plant residues and brown coal. We experimentally obtained the dependence of the yield of volatile substances from peat particles on residence time in a drop tube furnace. To achieve the almost complete release of volatile substances from particles of size 0.20–0.25 mm, 450 ms was sufficient. The results obtained allow us to predict the nature of processes occurring in the low-temperature combustion chambers of industrial heat-generating plants, where the temperature level is close to the tested one. The thermogravimetric analysis of peat charcoal obtained upon the pyrolysis of peat in a drop tube furnace showed that the charcoal after a more prolonged heat treatment ignited at higher temperatures; in this case, the height of the first peak characteristic of the ignition and combustion process of volatiles decreased, and this peak was almost absent at an exposure time of 450 ms. For the second peak characteristic of the process of ignition and combustion of the char residue, the opposite was observed. The temperature and time intervals of the combustion process of peat charcoal increased, as compared with those of the original peat. The temperatures characteristic of the first peak maximum increased with the time of the heat treatment of peat, and those of the second peak decreased. This fact indicates that the reaction properties of peat charcoal increased with the duration of the high-temperature processing of peat in an inert atmosphere to increase its burnout rate. The lower the volatile content of the peat charcoal, the greater the similarity of DTG curves obtained on the combustion of charcoals and coals. 288 MARYANDYSHEV et al. REFERENCES 1. Perrone, D., Klimanek, A., Morrone, P., and Amelio, M., Fuel Proc. Technol., 2018, vol. 181, p. 361. 2. Echi, S., Bouabidi, A., Driss, Z., and Abid, M.S., Energy, 2019, vol. 169, p. 105. 3. Long-fei, Z., Hong-zhou, H., and Huang-huang, Z., Energy Procedia, 2014, vol. 61, p. 2026. 4. Wang, C., Liu, Y., Zheng, S., and Jiang, A., Energy, 2018, vol. 153, p. 149. 5. Madejski, P., Appl. Thermal Eng., 2018, vol. 145, p. 352. 6. Karim, R. and Naser, J., Fuel, 2018, vol. 222, p. 656. 7. Meloni, E., Caldera, M., Palma, V., Pignatelli, V., and Gerardi, V., Renew. Energy, 2019, vol. 131, p. 745. 8. Gomez, M., Martin, R., Chapela, S., and Porteiro, J., Energy Convers. Manage., 2019, vol. 179, p. 91. 9. Moco, A., Costs, M., and Casaca, C., Energy Convers. Manage., 2018, vol. 169, p. 383. 10. Botelho, T., Costa, M., and Wilk, M., Magdziarz, Fuel, 2018, vol. 212, p. 95. 11. Colom-Diaz, J., Alzueta, M., Fernandes, U., and Costa, M., Fuel, 2017, vol. 207, p. 790. 12. Zellagui, S., Trouve, G., Schonnenbeck, C., ZouaouiMahzoul, N., and Brilhac, J.-F., Fuel, 2017, vol. 189, p. 358. 13. Costa, F. and Costa, M., Fuel, 2015, vol. 159, p. 530. 14. Zhang, K., Wang, Z., Fang, W., He, Y., Hsu, E., Li, Q., Gul-e-Rana, J., and Cen, K., J. Anal. Appl. Pyrolysis, 2018. 15. Zellagui, S., Schonnenbeck, C., Zouaoui-Mahzoul, N., Leyssens, G., Authier, O., Thunin, E., Porcheron, L., and Brilhac, J-F., Fuel Proc. Technol., 2016, vol. 148, p. 99. 16. Keller, F., Kuster, F., and Meyer, B., Fuel, 2018, vol. 218, p. 425. 17. Zhong, S., Baitalow, F., and Meyer, B., Fuel, 2018, vol. 234, p. 473. 18. Afanas’ev, A.E., Inisheva, L.I., and Kovalev, N.G., Kontseptsiya okhrany i ratsional’nogo ispol’zovaniya torfyanykh bolot Rossii (Concept of the Protection and Rational Use of Peat Bogs in Russia), Tomsk: TsNTI, 2005. 19. Hu, Z., Christensen, E., Restuccia, F., and Rein, G., Proc. Combust. Inst., 2018. 20. Jiuling, Y., Naian, L., Haixiang, C., Wei, G., and Ran, T., Proc. Combust. Inst., 2018. 21. Huang, X., Rein, G., and Chen, H., Proc. Combust. Inst., 2015, vol. 35, p. 2673. 22. Cancellieri, D., Leroy-Cancellieni, V., Leoni, E., Simeoni, A., Kuzin, A., Filkov, A., and Rein, G., Fuel, 2012, vol. 93, p. 479. 23. Kassman, H., Pettersson, J., Steenari, B.-M., and Amand, L.-E., Fuel Proc. Technol., 2013, vol. 105, p. 170. 24. Ohenoja, K., Korkko, M., Wigren, V., Osterbacka, J., and Illikainen, M., J. Environ. Manage., 2018, vol. 206, p. 607. 25. Popova, E., Chernov, A., Maryandyshev, P., Brillard, A., Kehrli, D., Trouve, G., Lyubov, V., and Brilhac, J.F., Bioresource Technol., 2016, vol. 218, p. 1046. Translated by V. Makhlyarchuk SOLID FUEL CHEMISTRY Vol. 53 No. 5 2019