1998
Energy & Fuels 2009, 23, 1998–2005
Investigation of Coal Char-Slag Transition during Oxidation: Effect of Temperature and Residual Carbon Suhui Li* and Kevin J. Whitty Institute for Clean and Secure Energy, UniVersity of Utah, 50 South Central Campus DriVe, MEB Room 3290, Salt Lake City, Utah 84112 ReceiVed December 14, 2008. ReVised Manuscript ReceiVed February 2, 2009
The transition of coal char to molten slag at high conversion was studied for a bituminous coal using a laminar entrained-flow reactor under oxidizing conditions. Post-oxidized char particles were analyzed by various techniques including loss-on-ignition, gas adsorption analysis, and scanning electron microscopy to determine carbon content, internal surface area and pore size distribution, and char morphology, respectively. These analyses provide information concerning the effect of temperature and residual carbon on the transition from porous char to molten slag. Results showed that, at temperatures above the ash flow temperature, the transition from porous char to molten slag occurred at about 90% conversion for the coal used in this study. No transition occurred at temperatures below the ash flow temperature. This finding explains previous observations that there is a coal-dependent critical carbon conversion at which the ash stickiness increases dramatically. This result also indicates that surface area can be used as a criterion for determining the critical conversion of the transition. In addition, it was found that the randomly overlapping pore model cannot be directly applied to predict the surface area evolution of char particles during the transition without considering the reopening of closed micropores during the initial reaction and the ash fusion effect.
Introduction Ash deposition and slagging related issues are usually the major concern in the design and operation of coal-fired boilers and slagging coal gasifiers. The most important mechanism in ash deposition is inertial impaction.1 According to Baxter2,3 and Desollar,3 the collection efficiency (a measure of the ash deposition rate) of ash particles caused by inertial impaction is proportional to the particle capture efficiency, which depends upon the ash stickiness and the impacting surface property. Barroso et al.4 also showed that capture efficiency is representative of the intrinsic tendency of ash particles to deposit. Because the impaction surface usually remains essentially the same for a specific boiler or gasifier, ash stickiness plays a crucial role in determining if the ash particle will stick to or rebound from the impacting surface. Therefore, ash stickiness has been studied extensively in terms of ash chemistry, temperature, viscosity, and the fraction of molten phase. Issak et al.5 found that for synthetic ash the stickiness criterion is 10-20% weight fraction of the liquid phase in the particle. Experiments and thermodynamic calculations using synthetic ash by Tran et al.6 confirmed that the stickiness criterion for alkali-rich ash is 15% weight fraction of the molten phase. On the other hand, Srinivasachar * To whom correspondence should be addressed. Telephone: +1-801949-6986. E-mail:
[email protected]. (1) Strandstro¨m, K.; Mueller, C.; Hupa, M. Fuel Process. Technol. 2007, 88, 1053–1060. (2) Baxter, L. L. Biomass Bioenergy 1993, 4, 85–102. (3) Baxter, L. L.; Desollar, R. W. Fuel 1993, 72, 1411–1418. (4) Barroso, J.; Ballester, J.; Ferrer, L. M.; Jime´nez, S. Fuel Process. Technol. 2006, 87, 737–752. (5) Issak, P.; Tran, H. N.; Barham, D.; Reeve, D. W. J. Pulp Pap. Sci. 1986, 12, 84–88. (6) Tran, H. N.; Mao, X.; Kuhn, D. C. S.; Backman, R.; Hupa, M. Pulp Pap. Can. 2002, 103 (9), 32–33.
et al.7 found that, for a silica-rich ash particle with kinetic energy typical of coal-fired boilers, the stickiness criterion is a viscosity range of 105-108 Pa s. Various models8-11 have been developed for the prediction of ash deposition and slagging based on these two criteria. However, these criteria are derived from the properties of synthetic ash (pure inorganic minerals). Ash particles in a boiler or gasifier usually contain residual carbon, which inevitably affects the melting behavior, viscosity, and other properties influencing the stickiness of ash particle. Koyama et al.12 studied the ash deposits in a two-stage entrained-bed coal gasifier. They found that unburned char particles served as a dispersive material, which prevented complete sintering even at high temperatures, and this dispersive effect decreased as the content of residual carbon in the char decreased. Bool and Johnson13 studied the ash deposition behavior during coal combustion using an entrained-flow reactor at high temperature. They observed that the collection efficiency of the ash deposit on the deposition probe increased dramatically to a maximum value at a critical char burnout and then decreased slightly throughout the burnout process. This result, from a macroscopic view, suggests that the effective ash stickiness is dependent upon the (7) Srinivasachar, S.; Helble, J. J.; Boni, A. A. Proceedings of the 23rd International Symposium on Combustion, 1990; pp 1305-1312. (8) Lee, F. C. C.; Lockwood, F. C. Prog. Energy Combust. Sci. 1999, 25, 117–132. (9) Mueller, C.; Selenius, M.; Theis, M.; Skrifvars, B. J.; Backman, R.; Hupa, M.; Tran, H.; Naruse, I. Proc. Combust. Inst. 2005, 30, 2991–2998. (10) Ma, Z.; Iman, F.; Lu, P.; Sears, R.; Kong, L.; Rokanuzzaman, A. S. Fuel Process. Technol. 2007, 88, 1035–1043. (11) Wang, X. H.; Zhao, D. Q.; He, L. B.; Jiang, L. Q.; He, Q.; Chen, Y. Combust. Flame 2007, 149, 249–260. (12) Koyama, S.; Morimoto, T.; Ueda, A.; Matsuoka, H. Fuel 1996, 75, 459–465. (13) Bool, L. E.; Johnson, S. A. American Society of Mechanical Engineers, Environmental Control Division Publication, EC, 1, Environmental Control/Fuels and Combustion Technologies, 1995; pp 305-312.
10.1021/ef801082j CCC: $40.75 2009 American Chemical Society Published on Web 03/04/2009
Coal Char-Slag Transition during Oxidation Table 1. Properties of a Typical Illinois #6 Coal analysis Proximate Analysis (wt %, Dry) 2.54 moisturea ash 12.33 volatiles 39.40 fixed carbon 48.28 Ultimate Analysis (wt%, Dry Ash Free) carbon 78.91 hydrogen 5.50 nitrogen 1.38 sulfur 4.00 oxygen 10.09 chlorine 0.11 IT ST HT FT a
Ash Fusion Analysis (Oxidizing, °C) 1256 1265 1277 1342
As received.
residual carbon content. Furthermore, the sharp rise in the stickiness indicates a substantial change in the structure of ash particles around the critical burnout, possibly the transition from porous, nonsticky char to molten, sticky slag. On the other hand, at the later stage of gasification (above 90% conversion), the transition from porous char to nonporous slag affects overall carbon conversion because the decrease in porosity increases the resistance of reacting gas diffusing into the particle. Consequently, it is essential to understand the mechanism behind the sharp change of ash stickiness at the microscopic level, i.e., the structural changes during the char-slag transition. The surface area is one of the most important features for characterizing the microscopic structure of char and ash particles.14 Scanning electron microscopy (SEM) is a powerful tool to directly view the microscopic structure and morphology of particles. Laminar entrained-flow reactors (LEFRs) have been widely used to study coal char oxidation because of their ability to provide control of experimental conditions while closely representing the environment in practical coal combustion and gasification systems.4 In this work, char particles with various residual carbon contents were prepared in a LEFR under atmospheric pressure and at different temperatures that are typical of coal combustion conditions. Collected particles were characterized by loss-onignition (LOI), gas adsorption analysis, and SEM. These analyses provide insight into the structural changes associated with the transition from porous char to molten slag, as well as the effect of temperature and residual carbon on the transition. The indication of char-slag transition on ash stickiness and char conversion is discussed. Experimental Section Coal Sample. An Illinois #6 bituminous coal was selected for this study. The coal was pulverized and sieved to 104 µm or less. Properties of a typical Illinois #6 coal are presented in Table 1 for reference. Experimental Apparatus and Procedures. The LEFR used in this study for char and ash preparation is shown in Figure 1. It consists of a high-temperature furnace, a coal feeder, a sample collector, gas supply, and cooling water circulator. Two coaxial alumina tubes (89 mm outer diameter × 75 mm inner diameter × 1500 mm long and 57 mm outer diameter × 50 mm inner diameter × 1000 mm long, respectively) are mounted vertically inside the (14) Bar-Ziv, E.; Kantorovich, I. I. Prog. Energy Combust. Sci. 2001, 27, 667–697.
Energy & Fuels, Vol. 23, 2009 1999 furnace (Carbolite, single zone, 1600 °C maximum operation temperature, and 610 mm heated length) and sealed with flanges. The inside tube is used as the reactor. The reaction gas (a premixed air-nitrogen mixture) is injected through three injection ports on the bottom flange and is preheated when it flows upward through the annulus between the two coaxial tubes. When the reaction gas reaches the top of the annulus, it turns and flows down into the inner tube through an alumina honeycomb flow straightener. The flow straightener has a sufficient pressure drop to generate a uniform and laminar flow, which is essential so that the entrained particles can travel along the centerline of the reactor tube experiencing identical reaction conditions. Coal particles are fed into the reactor through an injection probe using a vibrating syringe pump type coal feeder with nitrogen as carrier gas. The injection probe is watercooled to prevent the coal particles from being heated before reaching the reaction zone. Upon injection into the reactor, the coal particles undergo pyrolysis and react with the reaction gas to produce char particles. The reacting products exit the reactor through a water-cooled collection probe. Nitrogen is injected into the collection probe through a sintered stainless-steel tube to quench the product stream and reduce the thermophoresis deposit of the char particles on the cold surface of the probe. Char particles are collected using a cyclone separator followed by a filter. Experimental Conditions. For the experiments performed in this study, the pressure inside the reactor was maintained at local atmospheric pressure, 0.84 atm (the altitude of Salt Lake City is about 1500 m). Experiments were carried out at four temperatures (1200, 1300, 1400, and 1500 °C), which cover the range below and above the ash fusion temperatures. Two kinds of experiments were performed: devolatilization experiments and oxidation experiments. In the devolatilization experiments, the residence time was fixed at 2 s. The devolatilization gas was pure nitrogen, which provided an inert gas environment. The residence time was controlled by adjusting the flow rate of devolatilization gas at different temperatures. The flow rate of coal carrier gas flowing through the injection probe was set to maintain the same velocity as the devolatilization gas in the reactor. In the oxidation experiments, the residence time was varied from 1.5 to 6 s to achieve various carbon conversions. The use of a long residence time was due to the low oxygen content (1.8-5.7%) in the reaction gas for the very low feeding rate (20 mg/min) of the coal sample. The reaction gas was an air-nitrogen mixture. The residence time was controlled by adjusting the flow rate of nitrogen while fixing the flow rate of air in the reaction gas. This also varied the oxygen concentration in the reaction gas. The flow rate of the air in the reaction gas was set to be 0.24 standard liter per minute (SLPM), which was 50% more than necessary for complete combustion of coal. The flow rate of the coal carrier gas was set to maintain the same velocity as the reaction gas in the reactor. The flow rate of nitrogen in the reaction gas was equal to the total gas flow rate minus the air and the carrier gas rates in the reactor. Under these conditions, the Reynolds number of the gas flow inside the reactor was below 50, which ensured a laminar flow. Calculation of the terminal velocity (
