Effect of gasifying agents and their partial pressure on the gasification rate of Shengli brown coal Shu Zhang, Xujun Chen, Xueying Zhang, Yonggang Wang*, Deping Xu School of Chemical and Environmental Engineering China University of Mining and Technology (Beijing), Beijing 100083, China *Corresponding author: wyg1960@126.com Email: zhangshuwo@hotmail.com Tel:+86-10-62331048 Abstract: This study aims to investigate the changes in coal conversion during the gasification of Shengli brown coal (one of the most abundant Chinese lignites) in steam and steam+oxygen in a drop tube reactor. The steam concentration (partial pressure) was varied in the absence and presence of low concentration of oxygen. Reactivity of some chars from the gasification was further characterized using TGA (thermogravimetric analyse) and Na retention/concentration in char was also determined. The results indicate that the mixed gasifying agents of steam and oxygen (low concentration) had strong synergistic effects on char converting rate in the drop tube reactor, but heavily relying on the steam concentration. The change in Na retention and concentration in char could not fully explain the changes in char yields and char reactivities. The interactions of steam and oxygen on char surface may have significantly induced the change in char structure. Key words: gasifying agents; partial pressure; lignite; drop tube reactor; Ⅰ. INTRODUCTION Among various coal utilization technologies, coal gasification offers especially higher energy efficiency and is well known for its lower environmental impacts [1-4]. Achieving high gasification efficiency is one of the most important aspects to facilitate the industrialization of IGCC (integrated gasification combined cycle) system. Brown coal is very reactive materials for gasification due to its high amorphous carbon structure and rich oxygen contents. The reactivity of coal/char is highly determined by both carbon structure and AAEM (alkali and alkaline earth metallic species) in the coal/char. The AAEM species could be dispersed to atomic scales by ion exchange functionality of the oxygen-containing groups in brown coal [5]. According to Jamil s study [6], nascent char from rapid pyrolysis (at a heating rate of 1000 K/s) has extremely high reactivity for steam gasification. A higher heating rate causes more rapid thermal cracking, forming more radicals in the nascent char and providing more active sites for the steam gasification [7]. In a very short period, the AAEM species may quickly catalyze gasification of nascent char, as well as reform fresh volatiles on the char surface. As gasification in steam produces the most desirable gases of CO and H 2, a number of studies for examining the effect of steam concentration on the char structure and the content of AAEM species using fixed/fluidized bed reactor have been carried out to date [5, 8-9]. Among all the AAEM species, sodium has the most attention due to its activity and content in coal [9]. Although heating rates from fixed/fluidized bed can be very high, the residence time of chars inside reactors are much longer than that of entrained flow gasifier which is one of most popular gasification reactors nowadays due to its flexibility to feedstock and high gasifying efficiency. In a drop tube, the coal particles and gasifying agents are fed into reactor co-currently at the target temperature. The heating rate and residence time of particles could be very similar to those in an entrained flow gasifier. Furthermore, in a practical
gasifier, low concentration of oxygen is always present in gasification zone. Thus, this study is to investigate the effect of steam concentration on lignite conversion in a newly-designed drop-tube reactor in the absence and presence of low concentration of oxygen. The reactivity of char prepared from the reactor was further analyzed using TGA (thermogravimetric analyse). The trends in coal and char converting rates in the drop tube reactor and TGA were explained by considering Na contents as well as char structure. Ⅱ. EXPERIMENTAL A. Coal samples Shengli brown coals were pulverized, sized to a range from 61 to 98 μm, dried at 328 K for 48 h under vacuum condition, and then stored in a desiccator before subjected to experiments. The ultimate and proximate analyses of coal are listed in Table 1. Table 1. Ultimate and proximate analyses of coal used was the key experimental variable in this study, and those values were 0, 10%, 15%, 20%, 25%, 33%, respectively. It should be noted that the final reaction temperature was always achieved no matter which steam concentration was used before coal feeding started. The total gas flow and the gas velocity were 5.67 L/min and 2 m/s at 1173 K and atmospheric pressure, respectively. After the temperature reached the desired temperature, about 4g of coal sample was fed through a water-cooled injection probe at a rate of 80 mg/min into the reaction zone. Char was formed in the reaction zone and was collected by a char filter at the bottom of the reactor. The char filter was outside of heating zone. After coal feeding was finished, the gasifying agents were stopped, only argon continued to purge the reactor for cooling down. Chars were then collected for further analyses. Ultimate analysis(wt%, db) Proximate analysis(wt%, daf) M A VM FC C H O a N S 4.06 8.20 40.35 47.4 64.20 4.85 29.32 1.19 0.44 db, dry basis; daf, dry-ash-free; a, by difference B. Char preparation from partial gasification Char preparation experiments were conducted using a newly-designed drop-tube quartz reactor shown in Fig.1. The quartz reactor is 20 mm in inner diameter and 1300 mm in length. There are two inlets at the top of the reactor. A thermocouple was inserted into one of the inlets to monitor reaction zone s temperature. The reactor was heated up externally by an electrical furnace and kept at 1173K in this study. The flow rates of argon/oxygen were controlled by mass flow controllers. The concentration of oxygen was 1.6% by volume. The main gasifying agent entered the reactor from the other inlet. The water flow rate for steam gasification was controlled accurately by a peristaltic pump ( Longer Pump YZ1215X ). When the reaction zone s temperature reached 973K, water was fed into the reactor through a stainless steel tube. Steam was then quickly generated as the gasifying agent inside the reactor and mixed with argon/oxygen gas flow immediately. The steam concentration in total supplied gas Fig.1.Schematic diagram of the drop-tube reactor. C. Reactivity measurement A Perkin-Elmer Pyris 1 thermogravimetric analyse (TGA) with high temperature furnace was used to measure the reactivity of the chars from the partial gasification in the drop tube reactor. About 10 mg of char was loaded into a sample
pan and heated from ambient to 383 K in nitrogen (Ultra High Purity) and held for 30 min in order to fully remove moisture. This weight was taken as the dry weight of char. Then the char was heated at 50 K/min to the 593 K (which was determined by trial and error method to avoid ignition once air was introduced) in nitrogen, then the atmosphere was switched to air and the reactivity measurement commenced. The reactivity, R, was calculated by: R=- 1 dw (1) W dt where W is the char weight (daf ) at any given time t. When the weight loss almost leveled off, the temperature was increased to 873 K and held for 30 min in order to burn off any remaining carbonaceous material. The temperature of 873 K was selected to avoid any possible loss of ash or metallic metals at higher temperatures [10, 11]. The resultant mass was considered as the weight of ash. D. Na quantification About 100 mg of char was ashed in a muffle furnace at 873 K with 200 min holding time in the air. The rate of the heating was about 1K/min in order to ensure complete oxidation but without ignition. The ash was then digested with a mixture of HNO 3 and HF solution (1:1 by volume). After evaporating HNO 3 and HF, the sample was re-dissolved in high purity of deionized water for injection into an inductive coupled plasma emission spectrometer (Thermo Scientific icap 6000) to quantify Na. The Na content in coal was determined in the same way. The concentration and retention of Na in char were calculated as follows: Na concentration in char = (amount of Na in the char, mg) / (char, g) (2) Na retention = (amount of Na in the char, mg) 100 / (amount of Na in coal used, mg). (3) Ⅲ. Results and discussion A. Effects of steam concentration on char yields with/without oxygen C h a r Y ield,w t% (d b ) 55 50 45 40 35 30 Steam+O 2 balanced with Ar Fig. 2. Char yields as a function of steam concentration during gasification in the absence and presence of oxygen at 1173 K. However, when the steam concentration increased from 15% to 33%, the char yields during gasification in steam+o 2 decreased sharply, and then quickly increased. With the increase in steam concentration, more steam would absorb on char surface, providing more opportunities for the possible synergistic effects of steam/oxygen-char reactions. The steam-char and oxygen-char reactions might create activated sites for each other. With further increasing steam concentration till 33%, the steam could probably have occupied more active sites, thus reducing the oxygen-char reactions. Also, the steam-char reactions may lower down the surface temperature of char particles although the temperature of gas phase inside reactor may not significantly change considering the very low coal feeding rate in this study. The exact reason for the curve peak remains unknown, more work needs to be conducted for a better understanding, such as monitoring the temperature of reaction zone, and analyzing the char properties (e.g. surface area and porous structure). B. Na concentrations and retentions in chars. Fig. 3 and Fig.4 show the changes in the concentrations and the retentions of Na in chars with increasing steam
concentration. As shown in Fig.3 and Fig.4, for the chars produced from steam gasification, the concentration of Na slightly decreased with increasing steam partial pressure. But for the chars produced from the gasification in steam + O 2, the concentration of Na initially decreased with the steam concentration increased from 0 to 20%. However, the concentration of Na increased as the steam concentration increased from 20% to 33%. Broadly, the changes in Na retention and concentration for the chars were in agreement with the changes in char yields as shown in Fig.2. More coal/char conversion meant lower Na retention and concentration. The lowest point for Na concentration exists at 20% steam concentration while the lowest char yield was shown at about 25% steam concentration, implying that the synergistic effect from steam and oxygen have somehow released Na without gasifying char to some extent. Na Concentration in Char,mg/g(db) 21 20 19 18 17 16 15 14 13 Steam+ O 2 balanced with Ar Fig. 3. Concentrations of Na in the chars as a function of steam concentration N a R etention,% 70 60 50 40 30 Steam+ O 2 balanced with Ar Fig. 4. Retention of Na in the chars as a function of steam concentration C. Char reactivity in air In order to obtain more information of char properties, some of chars reactivity in air were measured using TGA at relatively low temperature of 593 K. Fig. 5(a) and (b) shows the reactivities of chars after the partial gasification at 1173 K in steam and in steam + O 2, respectively. The chars from 0% steam in both cases are showing higher reactivity than other chars. The Na concentrations from Fig.3 could not explain this phenomenon. Besides catalytic species contributing to the char reactivity, char structure was another important factor determining the relative ease of char oxidation reactions. The high char yields from the 0% steam gasification indicate that more amorphous/reactive carbon were left, thus show high reactivity as shown in Fig.5. The chars from 15% steam gasification show quite low reactivity, which also could not be elucidated by the catalytic metal species as the Na contents at the points were not low. Therefore, the char structure again may play importance roles for the observed changes in char reactivity. The synergistic effects from steam and oxygen actually started to take place when the steam concentration was around 15%. The interactions between steam and oxygen on the char surface could likely alter carbon skeleton structure by creating more amorphous carbon with abundant O-containing functional groups. The information of char structure will be further analyzed to confirm the assumption. Again, the change in temperature due to the variation of steam concentration was ignorable because the reaction temperature
was always obtained with supply of steam into the reactor before coal feeding. Secondly, as mentioned before, the coal feeding rate was too low to alter the reaction temperature to certain degree so that it would impact property of prepared chars. Specific Reactivity,m in -1 Specific R eactivity,m in -1 0.020 0.016 0.012 0.008 0.004 15% 25% 0% 0.000 0 15 30 45 60 75 90 Char Conversion,wt% (db) (a) Chars prepared in steam balanced with Ar 0.015 0.010 0.005 0% 25% 0.000 0 15 30 45 60 75 Char conversion,wt% (db) (b) Char prepared in steam+ O 2 balanced with Ar Fig. 5. Char reactivities measured in air in TGA at 593 K. The chars were prepared from partial gasification at 1173 K in (a) steam (balance) or (b) steam +O 2 (balance). The numbers on the curves denote the steam 15% concentrations during char preparation at 1173 K. Ⅳ. CONCLUSIONS This study aimed to examine the effect of the steam 33% concentration/partial pressure on the char gasification rates in the absence and presence of low concentration of oxygen. It was found that the synergistic effect of steam and oxygen has dramatically reduced char yields when the steam concentration was about 25%. The Na concentration in chars could not be fully responsible for the change in char conversion and char reactivity measured by TGA. The interactions of steam and oxygen on the surface of char particles must have initiated the change in char/carbon structure, thus affecting the char reactivity in steam and oxygen. ACKNOWLEDGMENT The authors gratefully acknowledge the support of this study by 12th Five-Year Plan of National Science and Technology Support (2012BAA04B02). REFERENCES [1] S. Yasushi, Reactivity and structural change of coal char during steam gasification, Fuel, vol. 85, 2006, pp. 122-126. [2] S.WU, The reactivity and kinetics of Yanzhou coal chars from elevated pyrolysis temperatures during gasification in steam at 900-1200, Process Safety and Environmental Protection, vol. 84, 2006, pp. 420-428. [3] P.Panaka, The utilization of Indonesia s low rank coal: its potential,challenges and prospects, Proceedings of the 22nd International Technical Conference on Coal Utilization and Fuel Systems, 1997, pp. 37-46. [4] C.Z. Li, Special issue gasification:a route to clean energy, Process Safety and Environmental Protection, vol. 84, 2006, pp. 407-408. [5] H.L. Tay, S. Kajitani, S. Zhang, C.Z. Li, Effects of gasifying agent on the evolution of char structure during the gasification of Victorian brown coal, Fuel, vol. 103, 2013, pp. 22 28. [6] K.Jamil, J.I. Hayshi,C.Z.Li, Prolysis of a Victorian brown coal and gasification of nascent char in CO 2 atmosphere in a wire-mesh reactor, Fuel 83(2004) 833-843. [7] L.X. Zhang, Catalytic effects of Na and Ca from inexpensive materials on in-situ steam gasification of char from rapid pyrolysis of low rank coal in a drop-tube reactor, Fuel Processing Technology, vol. 113, 2013, pp. 1-7. [8] C.Z. Li, Fates and roles of alkali and alkaline earth metals during the pyrolysis of a Victorian brown coal, Fuel, vol. 79, 2000, pp. 427-438. [9] H. Wu, J.I. Hayashi, T. Chiba, T. Takarada, C.Z. Li, Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal, Part V. Combined effects of Na concentration and char structure on char reactivity, Fuel, vol. 83, 2004, pp. 23 30. [10] D. M. Quyn, J.I. Hayashi, C.Z. Li, H. Wu, Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IV. Catalytic effects of NaCl and ion-exchangeable Na in coal on char reactivity, Fuel, vol. 82, 2003, pp. 587-593. [11] S. Zhang, J.I. Hayashi, C.Z. Li, Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal Part IX. Effects of volatile-char interactions on char-h2o and char-o2 reactivities, Fuel, vol. 90, 2011, pp. 1655-1661.