794
Energy & Fuels 2003, 17, 794-795
Communications Parametric Sensitivity Study of a CFD-Based Coal Devolatilization Model D. Gera,*,† M. Mathur,‡ and M. Freeman‡ Fluent Incorporated, 3647 Collins Ferry Road, Morgantown, West Virginia 26505, and USDOE National Energy Technology Laboratory, 626 Cochrans Mill Road, Pittsburgh, Pennsylvania 15236-0940 Received December 13, 2002 The true rate of coal devolatilizaton is a matter of some contention, and a number of approaches have been proposed to model the complex devolatilization process, ranging from simple Arrhenius expressions based on global kinetics6,7 to very complicated functional groups, depolymerization, vaporization, and cross-linking (FGDVC) pyrolysis models. The simple Arrhenius models correlate the weight loss (volatile yield) with the temperature in one or multiple steps.5 The FG-DVC model characterizes the devolatilization behavior of rapidly heated coal on the basis of the physical and chemical transformations of the coal structure (see, for example, ref 1). Recently, Jones et al.4 provided a detailed discussion on various devolatilization models and numerically simulated a drop tube furnace using the various sub models. Jones et al.4 showed that the single-step devolatilization rates gave satisfactory profiles. Several devolatilization kinetic rates, varying from 1 s-1 to 1000 s-1, are reported in the literature. These rates were derived assuming a single first-order process [k ) A exp(-E/RT), where A is the frequency factor, E is the activation energy, T is the temperature, and R is the gas constant] to define weight loss or tar evolution at 800 °C.4,7 Solomon et al.7 reviewed a list of parameters, namely, the heating rate, mass transfer rate, radiance, * Corresponding author. Tel.: (304) 598-7934. Fax: (304) 598-7185. E-mail:
[email protected]. † Fluent Incorporated. ‡ USDOE National Energy Technology Laboratory. (1) Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Grant, D. M. Chemical Percolation Model for Devolatilization: 2. Temperature and Heating Rate Effects on Product Yields. Energy Fuels 1990, 4, 54. (2) Gera, D.; Mathur, M.; Freeman, M. C.; O’Dowd, W. Moisture and Char Reactivity Modeling in Pulverized Coal Combustors. Combust. Sci. Technol. 2001, 172, 35-69. (3) Gera, D.; Mathur, M.; Freeman, M. C.; Robinson, A. Effect of Large Aspect Ratio of Biomass Particles on Carbon Burnout in a Utility Boiler. Energy Fuels 2002, 16 (6), 1523-1532. (4) Jones, J. M.; Patterson, P. M.; Pourkasanian, M.; Williams, A.; Arenillas, A.; Rubiera, F.; Pis, J. J. Modeling NOx Formation in Coal Particle Combustion at High Temperature: An Investigation of the Devolatilization Kinetic Factors. Fuel 1999, 78, 1171. (5) Kobayashi, H.; Howard, J. B.; Sarofim, A. F. (1976), Coal Devolatilization at High Temperatures. 16th International Symposium on Combustion; The Combustion Institute: Pittsburgh, 1976; pp 411-425. (6) Niksa, S.; Liu, G.; Hurt, R. H. Coal Conversion Submodels for Design Applications at Elevated Pressures. Prog. Energy Combust. Sci., in review. (7) Solomon, P. R.; Serio, M. A.; Suuberg, E. M. Coal Pyrolysis: Experiments, Kinetic Rates and Mechanisms. Prog. Energy Combust. Sci. 1992, 18, 113-220.
Figure 1. (a) Effect of activation energy on flame characteristics. (b) Effect of activation energy on temperature distribution inside the pilot combustor.
particle emissivity, heat capacitance, heats of reaction, and uncertainty in the measurement of temperature on wide variance of the kinetic rates. The objective of this paper is to present the effect of kinetic rates on the flame structure and its anchoring. Three cases are studied by varying the activation energy (E) ( 12.5% from the mean
10.1021/ef020286o CCC: $25.00 © 2003 American Chemical Society Published on Web 04/02/2003
Communications
value of 4.0 × 107 J kg mol-1, and assuming the frequency factor (A ) 1.8 × 105 s-1) to be the same for all the three cases. The initial CFD simulations were conducted on a pilot combustor at the National Energy Technology Laboratory (NETL) with a hexahedral grid of 80 000 cells, with an estimated volatile yield of 35% for Pittsburgh No. 8 coal. The combustor is designed to achieve similarity with fullscale boilers, including replicating typical specification ranges for burner relative mass flow, radiant furnace temperature distributions, gas residence time, and convective section gas velocity. The unit is a down-fired radiant combustor (52-cm internal diameter), with a transition chamber, and a horizontal convective section (152-cm2 internal passage). For additional details on coal properties and the combustor, see refs 2 and 3. To study the effect of three different activation energies on flame root position or the standoff distance from the burner, the temperature contours on the central radial plane are plotted along the length of the combustor in Figure 1a. Additionally, the predicted temperature values averaged over the radial plane of the combustor are compared with the experimentally measured values of temperature along the central axis for these three cases in Figure 1b. The temperature contours depict the flame to be approximately 0.75-0.9 m long, which is in excellent agreement with the visual observations during the experiments.2 The standoff distance or flame root position is determined by inserting a thermocouple from the top of the combustor to a point where the temperature was read as 800 °C (1073 K). It may also be observed from Figure 1b that the standoff distance increased from 0.36 to 0.54 m when the activation energy increased from 3.5 × 107 to 4.5 × 107 J kg-1 mol-1. The phenomena of flame liftoff may also be seen from the contours of temperature (CaseIII) in Figure 1a. Flame root position is an important parameter in the design of low NOx burnerssif the flame root is closer to the burner then it produces less NOx compared to the flame root away from the burner. This may be due to the fact that if the flame is detached from the burner then the concentration of oxygen near the flame-envelope increases the temperature locallysthereby increasing thermal NOx. Also, the excess (or unshielded from the flame) O2 near the flame-envelope results in production of fuel NOx. Jones et al.4 also reported similar NOx observations in numerical simulations of a lab scale combustor for several devolatilization rates. It should be noted from Figure 1b that the temperature farther downstream of the flame does not change considerably for three cases, and it matches well with the
Energy & Fuels, Vol. 17, No. 3, 2003 795
Figure 2. Effect of activation energy (Case-I: 3.5 × 107, CaseII: 4.0 × 107, and Case-III: 4.5 × 107 J kg mol-1) on mass loss along the length of the combustor.
experimental data. The temperature measurements were not taken near the burner because the hot temperature inside the fireball melted the temperature probes. Additionally, the high level of turbulence/swirl of product gases causes uncertainty in the temperature measurements near the flame. Volatile yields and devolatilization kinetic rates are typically derived from the time-resolved weight losses at several locations in drop tube furnaces. Appropriate values of the frequency factor (A) and of the activation energy (E) are then determined by performing a curve fit on the weight loss data. The weight loss with a variation of (12.5% from the mean activation energy (4.0 × 107 J kg mol-1) is shown for three cases in Figure 2. It is plausible that the activation energy obtained by determining a best curve fit from the weight loss is within 10-15% from the experimental data; consequently, it may have a considerable effect on the flame root position, flame length, and NOx. Summary It should be emphasized that errors in coal devolatilization parameters will have the dominant effect on predicted carbon burnout, local gas temperature, flame anchoring, and the NOx formation. A single-step mathematical model is used to describe the devolatilization process in the combustors. For optimal design and development strategies, it is essential to understand the devolatilization process as well as to be able to predict devolatilization time and products in combustors. EF020286O
