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Energy & Fuels 2004, 18, 908-912
Theoretical Study of Coal Gasification in a 50 ton/day HYCOL Entrained Flow Gasifier. I. Effects of Coal Properties and Implications Hao Liu† and Toshinori Kojima1* Department of Applied Chemistry, Seikei University, 3-3-1 Kichijojikita-machi, Musashino-shi, Tokyo 180-8633, Japan Received October 6, 2003. Revised Manuscript Received March 31, 2004
A three-dimensional computational fluid dynamics simulator was developed to study hydrogenfrom-coal (HYCOL) coal gasification theoretically. The simulator developed has satisfactory sensitivity to various coal properties, including C/H ratio, C/O ratio, volatile matter content, ash content, heating value, and particle size when simulating coal gasification in an oxygen-blown HYCOL gasifer. Among the coal properties investigated, the heating value had a strong influence on the gasification characteristics of a HYCOL gasifier, whereas the influence of particle size was limited, suggesting a very likely reaction regime of chemical reaction control in the gasification process. The trajectory and temperature distribution were quite different for particles of different sizes, which may be useful information to avoid ash slagging in the gasifier.
Introduction Hydrogen-from-coal (HYCOL) methodology is a promising technology for coal utilization. As a governmentsponsored energy program called the New Sunshine Project, a research and development project of HYCOL was conducted using a 50 ton/day oxygen-blown pilot scale plant.1-5 Theoretical study on entrained flow gasification is becoming more and more important, because of the difficulty and high cost of experimental study. Some researchers have developed several mathematical models for entrained flow gasifiers.6-9 These models have common features in regard to solving the mass, momentum, and energy conservation equations with similar submodels by almost the same mathematical methods.10 Some models have been applied to benchscale entrained flow gasifiers;11 however, the simulation * Author to whom correspondence should be addressed. Telephone: +81-422-37-3758. Fax: +81-422-37-3750. E-mail:
[email protected]. † Research fellow of NEDO (New Energy and Industrial Development Organization), Japan (1) Nogita, S.; Koyama, S.; Morihara, A. Development of HYCOL Technology. Sunshine J., (Jpn.) 1986, 7 (1), 70-77. (2) Ueda, F.; Yoshida, N.; Hashimoto, R.; Nomura, K. Characteristics of Oxygen-Blown Entrained-Bed Coal Gasifier Investigated in 50t/d Pilot Plant. Kagaku Kogaku Ronbunshu 1994, 20 (6), 766-773. (3) Ueda, F.; Yoshida, N.; Hashimoto, R.; Endo, M.; Nomura, K.; Kida, E.; Koyama, S. Development of the HYCOL Gasification. In Proceedings of the 13th EPRI Conference on Gasification Power Plants (San Francisco, CA, October 1994). (4) Koyama, S. Key Technologies of Entrained-Bed Coal Gasification System. Jpn. J. Multiphase Flow 1990, 4 (2), 101-110. (5) Honma, S.; Takeda, S.; Kitano, K.; Yoshida, R.; Koyama, S.; Ueda, A. Pressurized Gasification of Entrained Coal Char from Entrained Bed Gasifier. J. Jpn. Inst. Energy 1998, 77 (8), 785-792. (6) Wen, C. Y.; Chaung, T. Z. Entrainment Coal Gasification Modeling. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 684-694. (7) Govind, R.; Shah, J. Modeling and Simulation of an Entrained Flow Coal Gasifier. AIChE J. 1984, 30, 79-91. (8) Smoot, L. D.; Smith, P. J. Coal Combustion and Gasification; The Plenum Chemical Engineering Series; Plenum Press: New York, 1985. (9) Hill, S. C.; Smoot, L. D. A Comprehensive Three-Dimensional Model for Simulation of Combustion System: PCGC-3. Energy Fuels 1993, 7, 874-883.
capability for gasification was quite limited, because of their oversimplification on the chemistry. To simulate entrained flow gasification, we have developed a comprehensive three-dimensional coal gasification model with a multisolid progress variables (MSPV) method.12 The validity of the model has been proved by comparing the predicted results and experimental data in our previous studies for an air-blown integrated coal gasification combined-cycle (IGCC) gasifier.12-15 In our previous work,16,17 the validity of simplifying the model was examined by investigating the interaction between turbulent flow and reactions. Recently, this model was extended to the simulation of an oxygen-blown HYCOL gasifier. The gasification characteristics of coal in the HYCOL gasifiers can be very different from that in the IGCC gasifiers, because of the different atmosphere, temperature, etc. In the HYCOL gasifier, oxygen, instead of air, is used as a gasification oxidizer. The layout of the gasifier chamber is also different. Figure 1 shows a (10) Smoot, L. D. Fundamentals of Coal Combustion for Clean and Efficient Use. Coal Sci. Technol. 1993. (11) Smoot, L. D.; Brown, B. W. Controlling Mechanism in Gasification of Pulverized Coal. Fuel 1987, 66, 1249-1256. (12) Chen, C.; Horio, M.; Kojima, T. Numerical Simulation of Entrained Flow Coal Gasifiers, Part 1: Modeling of Coal Gasification in an Entrained Flow Gasifier. Chem. Eng. Sci. 2000, 55, 3861-3874. (13) Chen, C.; Miyoshi, T.; Kamiya, H.; Horio, M.; Kojima, T. On the Scaling-Up of a Two Stage Air Blown Entrained Flow Coal Gasifier. Can. J. Chem. Eng. 1999, 77, 745-750. (14) Chen, C.; Horio, M.; Kojima, T. Numerical Simulation of Entrained Flow Coal Gasifiers, Part 2: Effects of Operating Conditions on Gasifier Performance. Chem. Eng. Sci. 2000, 55, 3875-3883. (15) Chen, C.; Horio, M.; Kojima, T. Use of Numerical Modeling in the Design and Scale-Up of Entrained Flow Coal Gasifiers. Fuel 2001, 80, 1513-1523. (16) Liu, H.; Chen, C.; Kojima, T. Theoretical Simulation of Entrained Flow IGCC Gasifiers: Effect of Mixture Fraction Fluctuation on Reaction Owing to Turbulent Flow. Energy Fuels 2002, 16, 12801286. (17) Liu, H.; Chen, C.; Kojima, T. Interaction between a Turbulent Flow and Reaction under Various Conditions in Oxygen Blown HYCOL Gasifiers. Dev. Chem. Eng. Miner. Process. 2002, 11, 557-577.
10.1021/ef030162r CCC: $27.50 © 2004 American Chemical Society Published on Web 05/05/2004
Coal Gasification in an Entrained Flow Gasifier. I
Energy & Fuels, Vol. 18, No. 4, 2004 909
Table 1. Property of the Coala Proximate Analysis (As Received, wt %)
Ultimate Analysis (Dry, Ash Free, wt %)
moisture
ash
volatile matter, VM
fixed carbon, FC
C
H
N
O
S
5.3
12.1
46.7
35.8
77.6
6.5
1.13
13.9
0.22
a
The heating value of the coal is 33 170 kJ/kg.
gas and particle phases.8 The corresponding expressions of Φ, SΦ, SΦP, and µ have been described in our previous article.15 A Lagrangian method was used to track the solid particles, and the particle-source-in-cell model18 was used to address the interaction between gas and particle phases through various particle source terms. Turbulence in the gas phase was modeled by the two-equation k- model19,20 for closure. The particle motion and turbulent dispersion were simulated with the particle stochastic trajectory model. Coal devolatilization was modeled by a simple, two-step mechanism. The particle surface reactions were characterized by the 0.5-order multiple parallel reaction rate formulation of Smith.22 An extended version of the statistical coal gas mixture fraction model with the MSPV method23 was used. Four components of coal off-gas (i.e., four coal gas mixture fractions) were used to track the reaction products. These mixture fractions were defined as the mass ratio of coal off-gas to the total gas product and written as23
fi )
mi
(2)
4
min +
∑m
j
j)1
Figure 1. (a) Schematic diagram of a HYCOL gasifier. (b) Schematic overhead view of the nozzle geometry.
schematic diagram of a HYCOL gasifier. All these factors mean that the sensitivity and general applicability of the developed model should be clarified for the simulation of gasification in the HYCOL gasifier. The sensitivity of the developed model for simulating HYCOL gasifer was investigated by changing the coal properties in this work. The possible implication, such as the generation of H2-rich product gas through cogasification of a coal/biomass, was also investigated. The gasification charateristics clarified in this work could provide useful reference for the design and operation of practical HYCOL gasifiers. Basic Equations and Numerical Method The gas phase was assumed to be a steady-state, reacting continuum field that can be described by a general conservation equation as follows:8
∂ ∂ ∂ (FuΦ) + (FvΦ) + (FwΦ) ) ∂x ∂y ∂z ∂ µ ∂Φ ∂ µ ∂Φ ∂ µ ∂Φ + + + SΦ + SΦP (1) ∂x σΦ ∂x ∂y σΦ ∂y ∂z σΦ ∂z
(
) (
) (
)
where Φ refers to any quantity of mass, velocity components (u, v, and w), turbulent kinetic energy (k), turbulent kinetic energy dissipation rate (), gas enthalpy (h), mixture fractions (fi), and their variances (gi). SΦ is the source term, and SΦP is an additional source term representing the interaction between
The conservation equations were Favre-averaged and solved by the SIMPLER method.24 A line-by-line iteration technique was used to solve the finite difference equations. A 43 × 43 × 68 grid mesh was adopted. A converged solution was defined when the global energy balances to within 0.3% deviation of the total combustion energy, and the normalized residual (mean residual/mean inlet velocity) for each velocity component was
