Comparison of coal reactivityduring conversion into different oxidizing medium

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1 Journal of Physics: Conference Series PAPER OPEN ACCESS Comparison of coal reactivityduring conversion into different oxidizing medium To cite this article: A G Korotkikh et al 2016 J. Phys.: Conf. Ser View the article online for updates and enhancements. This content was downloaded from IP address on 18/11/2018 at 16:52

2 Comparison of coal reactivityduring conversion into different oxidizing medium A G Korotkikh 1, K V Slyusarskiy 1, K B Larionov 1 and V I Osipov 1 1 Department of Nuclear and thermal power plants, Tomsk polytechnic university, Tomsk, , Russia konstantinsv@tpu.ru Abstract. Acoal conversion process of different coal samples into three different types of oxidizing medium (argon, air and steam) were studied by means of thermogravimetry. Two coal types with different metamorphism degree (lignite and bituminous coal) were used. The experimental procedure was carried out in non-isothermal conditions in temperature range from 373 K to 1273 K with 20 K/min heating rate. Purge gas consisted of argon and oxidizer with volumetric ratio 1:24 and had 250 ml/min flow rate.the ignition and burnout indexes were calculated to evaluate sample reactivity at different oxidizing mediums. The highest reactivity coefficient values in same atmosphere were obtained for lignite. It was caused by higher particle special surface area and volatile matter content. 1. Introduction The different coal conversion processes are very important for all fuel-using technologies as it defines many properties: ignition temperature [1], char porosity and surface area [2], pyrolysis gas composition [3] etc. For combustion the pyrolysis and volatile matter release define process characteristics [2-3]. That s why the information on coal conversion in specific conditions for combustion (low pressure, excess amount of air with low water and carbon dioxide content, high heating rate) is widely represented in scientific literature [4]. On the other hand, the development of energy-efficient technologies like gasification or oxy-fuel combustion creates need for data on pyrolysis process characteristics in other conditions.gasification is process of turning solid fuel into combustible gas for further application [5]. The gasification is realized in oxidizing medium with low oxygen and high steam and carbon dioxide content [6] as well as at high pressures and relatively low temperatures [7]. The conclusion could be made that process properties in combustion and gasification conditions are significantly different. The lower process intensity determine higher interest for particle pore structure formation and surface area evolution during pyrolysis. The coal conversion processes for two coal samples in medium of argon, air and steam were studied by means of thermogravimetry as well as sample chemical composition and physical properties. Thetwo reactivity indexes ignition and burnout indexes were calculated. 2. Experimental section 2.1 Sample characterization Samples of two coal types were used: bituminous coal from Kuznetskiy deposit and lignite from Kansk-Achinsk deposit. Raw material was grinded in ball-mill and sieved through cribble with mesh size 80 µm. Fractional composition of produced powder was investigated via laser Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by Ltd 1

3 diffractometryanalyzer HELOS (Sympatec, Germany). As long as powder was produced by sieving through 80 µm the larger fraction was considered to have cylindrical form with diameter lower than 80 µm. The coal sample specific surface area values were defined by means of BET analysis usingnitrogen via 3Flex surface characterization analyzer (Micrometrics, USA) and also presented in table 1.The volumetric version of sorption method was used. Surface specific area was defined by nitrogen vapor low-temperature isotherm. Table 1. Coal samples properties. Solid fuel sample Sample surface specific area, m 2 /g Particle average diameter, µm Bituminous coal 2,6 69,31±0,29 Lignite 6,3 69,67±0,09 The coal proximate and ultimate analysis resultsare presented in table 2. The proximate and ultimate analysis data are given for sample dry mass. Table 2. Coal sample composition Solid fuel sample Proximate analysis, mass. % Ultimate analysis, mass. % M A V FC C H N S O Bituminous coal <1 4 Lignite <1 28 *The M refer to moisture, A to ash, V volatile matter, FC fixed carbon, C carbon, H hydrogen, N nitrogen, S sulfur, O oxygen. 2.2 Experimental procedure The 20 mg coal powder samples thermal analysis was made using simultaneous TG-DSC analyzer Netzsch STA 449 F3 Jupiter (Netzsch, Germany). The samples were heated into furnace with rate 20 ºC/min to 1000 ºC. To eliminate particle drying stage the process was studied in temperature interval from 200 to 1000 C.The heating rate was chosen empirically to supply reaction in wide temperature range while ensuring completeness of conversion at estimated temperature. Signal registration was started from 100 C to eliminate water evaporation phase from measurement results. The purge argon flow rate for all experiments was 10 ml/min (it was needed for correct operation of analyzer). For experiments in pure argon medium the purge gas flow was supplied with additional 40 ml/min of argon (at standard conditions), for experiments in steam medium 240 ml/min (or 140 mg/min) of steam, in air medium 240 ml/minof atmospheric air (at standard conditions). 2.3 Reactivity evaluation The coal reactivity was determined using ignition index as long as coal ignition appears on this stage according to [8]. To evaluate the further influence of formed by pyrolysis particle pore structure on oxidation kinetics the burnout index was calculated [8, 9]. The higher value the ignition and burnout indexes have the higher coal reactivity. The ignition index was calculated by equation as follows [8]: D vmax i = t t p e Here v max is maximum reaction rate, mass. %/s; t p is the moment of time corresponding to maximum reaction rate, s; t e is ignition time, s. The burnout index was defined by following equation [9]: vmax D f = t tt Here t1/2 1/2 p f is time period of reaction rate values which are higher than v /2 max, s; t f is burnout time, s. The ignition time values were defined according to presented in [9] methodic by TG-curves (fig.1). The ignition temperature and time corresponds to crossing point (B in fig.1) of tangent lines to TG- 2

4 curve at maximal rate (point A in fig.1) and initial level. Other time values were defined using DTGcurves (fig.2). These indexes characterize coal reactivity, the lower the ignition temperature and thehigher the ignition index and the burnout index, the better the reactivity. Calculation were made for the most developed stage.for better illustration of experimental results, relative reaction rate was calculated. 3. Results and discussion 3.1 TG and DTG curves The experimental TG and DTGcurves for lignite and bituminous coal in different mediums are presented in figures 1 and 2, respectively. The solid line refer to experiments in air medium, dash line for experiments in steam medium, das-dotted line for experiments in argon medium. a) b) Figure 1. Experimental TG curves for lignite (a) and bituminous coal (b). 3

5 a) b) Figure 2. Experimental DTG curves for lignite (a) and bituminous coal (b). The TG-curve analysis for sample oxidation in different mediums have the same form. It has one stage in argon and air atmosphere and two stages in steam. The argon curve situated above others and has the lesser slope that indicates a low reaction rateat all temperatures. The average rate for steam and air experiments are close to each other while the form is completely different. The curve for oxidation in air medium has one clear stage resulted into one curve slope. For steam oxidation it has two stages with lesser slope at temperatures lesser than 700 C and high slope within temperature range C. It is well seen on DTG-curves as well. Reaction rate for lignite samples in same conditions is higher than it for bituminous coal. It is connected to lesser carbon content in lignite samples. One more consequence is lower temperature of coal burnout. 4

6 3.2 Reactivity evaluation Coal reactivity was evaluated according to presented in paragraph 2.3 methodology. Results are presented in table 3. Table 3.Coal oxidation reactivity in different oxidizing medium. Coal sample Oxidizing medium Lignite Bituminous coal vmax v, , D max i 10, mcurrent t p, s t e, s t 1/2, s t f, s mass.%/s 1/s 2 1/s 1/s Argon 2,69 4, ,37 55,69 Air 3,53 5, ,67 16,63 Steam 11,96 58, ,91 257,01 9 D f 10, Argon 0,74 1, ,65 2,77 Air 2,32 3, ,31 4,88 Steam 5,66 87, ,47 16,93 The reactivity for lignite conversion processes is lower than for bituminous coal. It is connected to more developed pore structure of lignite particles and higher volatile matter content. It is true for both ignition and burnout indexes: their values for lignite are higher than for bituminous coal. This is further proved by close maximal reaction rate values for lignite in argon and air mediums. Different maximal reaction rate value points out its complex nature which consisted of carbon oxidation reaction and volatile matter release. The second process has larger contribution and caused higher burnout characteristics because of low carbon oxidation reaction impact and low t1/2 values. The situation for bituminous coal is similar but low volatile matter content resulted into lower burnout index. Because of different nature the processes in argon medium has ignition index values two times higher than for processes in air. This tendency could be seen for both coals. Burnout characteristics are better for processes with high substance content. That s why it is high for processes in argon with lignite and for chemical reactions with bituminous coal. In general, ignition temperature for both lignite and bituminous coal in argon and air atmospheres are close to each other. In steam they are significantly higher as well as maximal reaction rate. It s connected to higher activity of water molecules in interaction with carbon. 4 Conclusion The coal pyrolysis process in three different mediums (inert argon, air and steam) was investigated by means of thermogravimetry in temperature range from 200 to 1000 C with heating rate 20 K/min. The gas flow during experiments was 250 ml/min for steam and air, 60 ml/min for inert argon. The sample surface area, average particle size and coal composition were determined. The ignition and burnout indexes for all samples and processes were determined in order to evaluate coal reactivity in different processes. The lignite reactivity was higher than bituminous coal for all processes because of calculated indexes higher values. It may be connected to higher volatile matter content and more complex particle pore structure. The burnout index of samples during steam oxidation was higher than its during other experiments, especially in high temperature area, while ignition index for lignite was higher than for bituminous coal. It may be applied to improve fuel conversion process performance. 5 Reference [1] Gorlov EG, Andrienko V G, Nefedov K B, Lutsenko S V andnefedov BK2009 Solid Fuel Chem [2] Syrodoi SV, Kuznetsov GV andsalomatov VV 2015Solid Fuel Chem [3] Yang J, Chen H, Zhao W and Zhou J 2016J. Therm. Anal. Calorim [4] Hees J, Zabrodiec D, Massmeyer A, Habermehl M andkneer R 2016 Flow Turbul. Combust [5] Korotkikh A G and Slyusarskiy K V 2015 MATEC web conf

7 [6] Zhang B, Ren Z, Shi S, Yan S and Fang F 2016 Chem. Eng. Sci [7] Kelley MA, Jakulewicz MS, Dreyer CB, Parker TE, Porter J M2015 Rev. Sci. Instrum [8] Kang Y, Yu-Ming Z, Qing-Zhao Y, Cheng F and Ze-Wu Z 2012 Reac. Kinet. Mech. Cat [9] Li XG, Ma BG, Xu ZW and Wang XG2006 Thermochim. Acta Acknowledgement The study was realized in National research Tomsk polytechnic university in framework of federal target program Research and development in prior directions of scientific-technological complex development in Russia in year, unique identifier of R&D project RFMEFI58114X