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Theoretical Study of Coal Gasification in a 50 ton/day HYCOL Entrained Flow Gasifier. II. Effects of Operating Conditions and Comparison with Pilot-Scale Experiments Hao Liu† and Toshinori Kojima* 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
The sensitivity of our three-dimensional simulator was studied under various operating conditions. It was clarified that, in addition to coal properties, this simulator also has satisfactory sensitivity, with respect to the change in operating conditions. O2 partition between the lower and upper burners significantly influenced the temperature distribution in a gasifier. Staged gasification not only enhanced carbon conversion and cold gas efficiency, but also led to favorable temperature distribution that suppresses ash slagging in the gasifier. The carbon conversion increased as the O2/coal ratio increased, whereas the cold gas efficiency reached a maximum value as the O2/coal ratio increased. The carbon conversion and cold gas efficiency decreased as the load ratio of a gasifier increased, but with a limited magnitude. The predicted results were in good agreement with the experimental data obtained from a 50 ton/day hydrogen-from-coal (HYCOL) pilot-scale gasifier. The results obtained in this research may be useful information for the designing and operation of gasifiers. This simulator can be applied to predict the gasification characteristics of an oxygen-blown HYCOL gasifier.
Introduction In Part we showed that the simulator that we developed has satisfactory sensitivity to various coal properties, including C/H ratio, C/O ratio, volatile matter (VM) content, ash content, heating value, and particle size when simulating coal gasification in an oxygen-blown hydrogen-to-coal (HYCOL) gasifier. The importance of various factors of the coal properties was also investigated. However, in addition to coal properties, a simulator for the HYCOL gasifier must also have satisfactory sensitivity, with respect to the change in operating conditions, which may strongly influence the performance of the gasifier. It also will be helpful to clarify the most important factors influencing the overall characteristics of the gasifier. In addition, it is significant to clarify the optimum operating conditions through simulation, because it is very costly and difficult to derive them from experiments. Theoretical simulation also provides useful information to prevent ash slagging, a problem that must be solved for the reliable operation of practical HYCOL gasifiers. Tominaga et al.2 developed a simulator by incorporating an ash deposition submodel into the modern computational fluid dynamic code “FLUENT”. They studied the ash deposition behavior in the 50 ton/day HYCOL gasifier and achieved satisfactory agreement between
predictions and experimental results. Chen et al.3 theoretically studied the characteristics of the 200 ton/ day integrated coal gasification combined-cycle (IGCC) gasifier under various conditions, including various air ratios and coal types through comprehensive theoretical simulation. The effects of throat diameter ratios and swirl ratios of burner injectors were also studied. The effects of the gasifier geometry and jet configuration on
* 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) Liu, H.; Kojima, T. Theoretical Study of Coal Gasification in a 50 ton/day HYCOL Entrained Flow Gasifier. I. Effects of Coal Properties and Implications. Energy Fuels 2004, 18, XXXX-XXXX.
(2) Tominaga, H.; Yamashita, T.; Ando. T.; Asahiro, N. Simulator Development of Entrained Flow Coal Gasifiers at High Temperature and High-Pressure Atmosphere. IFRF Combust. J. 2000, (June), Article No. 200004. (ISSN 1562-479X.) (3) 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.
I,1
Figure 1. Effect of O2 partition on carbon conversion and cold gas efficiency, where the O2 partition ratio is defined as (O2 feeding rate of the lower burner)/(O2 feeding rate of the upper burner).
10.1021/ef030163j CCC: $27.50 © 2004 American Chemical Society Published on Web 05/05/2004
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Figure 2. Temperature profile versus O2 partition (in Kelvin), where the O2 partition ratio is defined as (O2 feeding rate of the lower burner)/(O2 feeding rate of the upper burner).
the flow hydrodynamics were examined to control the ash deposition on the gasifier wall.4,5 The swirl number for the multistage injection swirling gas flow was defined and proved to be the most important hydrodynamic scaling factor for the two-stage air-blown entrained flow IGCC gasifier. However, the characteristics of gasification in the HYCOL gasifier may be very different from the IGCC gasifier, because of the different atmosphere, geometry of the gasifier, layout of burners, etc. It is important to clarify the temperature distribution in the HYCOL gasifier because a high temperature in the gasification section is favorable, whereas a low temperature in the heat recovery section are favorable, to enhance the carbon conversion and cold gas efficiency, as well as to prevent ash slagging in the gasifier. So far, only limited studies on the HYCOL gasifier, particularly theoretical ones, have been reported. Therefore, in Part II, we have studied the effects of various operating conditionssincluding O2 partition, staged gasification, load ratio, and O2/coal ratioson the characteristics of the HYCOL gasifier. Method The sensitivity of the simulator was studied by changing various operating parameters or conditions. In this paper, all the conditions used in the calculation were kept the same as the base conditions listed in Table 1, except for those particularly specified. The same simulator and the related methods described in Part I1 were also adopted in this study (Part II). (4) 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. (5) 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.
Table 1. Base Conditions Used in the Calculation parameter particle size distribution 10% 10% 20% 20% 20% 20% mass mean diameter gas flow rate lower coal burner upper coal burner particle loading lower coal burner upper coal burner pressure
value 150 µm 100 µm 40 µm 20 µm 10 µm 4 µm 39.8 µm 0.34 kg s-1 0.180 kg s-1 0.289 kg s-1 0.289 kg s-1 3.0 MPa
In addition, the validity of the simulator was investigated by comparing the predictions and experimental data. The relative importance of various factors and the optimum conditions were studied and discussed, which may be very helpful information for the designing and operation of the oxygen-blown HYCOL entrained flow gasifier.
Results and Discussions Effect of O2 Partition. The effect of O2 partition was investigated by changing the feeding ratio of O2 between the lower burner and the upper burner. The O2 partition ratio was defined as the ratio of O2 feeding rate of the lower burner to that of the upper burner. As shown in Figure 1, both carbon conversion and cold gas efficiency had a maximum value, with respect to the O2 partition ratio. The case in which the O2 partition ratio was equal to 0.075/0.055 was the optimum condition, because the highest carbon conversion and cold gas efficiency were achieved. However, the effect of O2 partition on the carbon conversion and cold gas efficiency was not so
Coal Gasification in an Entrained Flow Gasifier. II
Figure 3. Effect of staged gasification on carbon conversion and cold gas efficiency: case a, all fed into the upper burner; case b, fed into both lower and upper burners ((the load of the lower burner)/(the load of the upper burner) ) (50%)/(50%)); and case c, all fed into the lower burner.
strong. Figure 2 shows the temperature profiles for various O2 partition ratios. The figure shows that the temperature distributions were significantly influenced by O2 partition ratios. The case in which the O2 partition ratio was equal to 0.065/0.065 was unfavorable, compared with other cases, because its high temperature in the heat recovery section, particularly near the throat of the gasifier, may cause ash slagging troubles. Moreover, the low temperature in the gasification section also causes low cold gas efficiency. Effect of Staged Gasification. For a HYCOL gasifier, the fuel and gases can be fed either into a single layer of burners or into several layers of burners; the
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later situation is called staged gasification. To clarify the effect of staged gasification, three casessi.e., all fed into the upper burner (case a), fed into both lower and upper burners (case b), all fed into the lower burner (case c)swere simulated. For case b, the load for the lower burner, relative to that of the upper burner ((load of the lower burner)/(load of the upper burner)) is (50%)/ (50%). The results shown in Figure 3 show that case b, with staged gasification, had higher carbon conversion and cold gas efficiency than case a or case c; i.e., the staging of gasification significantly improved the overall characteristics of the gasifier. Figure 4 demonstrated that the temperature profiles were much different for different cases. For cases a and c (i.e., when gasification was not staged), the temperature in the gasification section was too low to facilitate gasification, and the temperature in the heat recovery section was too high to prevent ash slagging. Case a had the lowest carbon conversion and cold gas efficiency, as well as the worst temperature distribution in the gasifier. In contrast, case b had a high temperature in the gasification section, which enhances gasification rate and accordingly high cold gas efficiency. Moreover, case b had a low temperature in the heat recovery section, which helps to prevent ash slagging on the wall of the gasifier. These results suggested that staged gasification has many merits, compared to nonstaged gasification. Effect of Load Ratio. The effect of load ratio on the carbon conversion and cold gas efficiency was shown in Figure 5. Here, the load ratio was defined as the ratio of load to basic load, based on the conditions listed in Table 1. When load ratio changed, the total amount of O2 and coal fed into the gasifier was changed simulta-
Figure 4. Effect of staged gasification on temperature profile (in Kelvin).
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Figure 5. Effect of load ratio on the carbon conversion and cold gas efficiency, where the load ratio is defined as (load)/ (basic load).
neously. It was found that both carbon conversion and cold gas efficiency decreased as the load ratio increased. Moreover, as shown in Figure 6, the temperature at a load ratio of 150% was higher than that at a load ratio of 50%. On the other hand, the average residence time of particles decreased approximately linearly as the load ratio increased (Figure 7). It seems that the high temperature at a load ratio of 150% partially offset the decrease in residence time of particles, compared to that at a load ratio of 50%, which accounted for the limited decrease in carbon conversion and cold gas efficiency from a load ratio of 150% to a load ratio of 50%. Comparing Figures 5-7, it seems that the carbon conversion and cold gas efficiency were more strongly influenced by the temperature in the gasifier than by the residence time of particles. Effect of O2/Coal Ratio and Comparison between Prediction and Experiment. Different cases of O2/
Liu and Kojima
coal ratio were calculated, to find the optimum operation conditions. The predicted results were compared with experimental data obtained from a 50 ton/day HYCOL gasifier (Figure 8). Both predictions and experimental data showed that, as the O2/coal ratio increased, carbon conversion increased. On the other hand, the cold gas efficiency increased first and decreased after attaining an O2/coal ratio of ∼0.75 (i.e., there was a peak for cold gas efficiency). This occurred because, when the O2/coal ratio was too low, the limited O2 suppressed the combustion of coal, and, accordingly, the low temperature in the gasifier led to a low cold gas efficiency. On the other hand, at high O2/coal ratios, gasification was suppressed by coal combustion, and, accordingly, the cold gas efficiency decreased. Up to an O2/coal ratio of 0.82, the carbon conversion increased monotonically as the O2/coal ratio increased, because O2 facilitated coal combustion and led to higher temperature. When the O2/coal ratio is >0.82, the carbon conversion does not change significantly, probably due to the limited residence time of particles in the gasifier. In addition, a high O2/coal ratio increases the flue gas volume and accordingly shortens the residence time of particles in the gasifier. Figure 8 suggests that our simulator can satisfactorily predict the gasification characteristics of an oxygen-blown HYCOL gasifier. Discussions In addition to the global characteristics of an oxygenblown HYCOL gasifier, such as carbon conversion, cold gas efficiency, etc., the distributions of temperature and gas concentration in a gasifier are also very important information, because ash slagging is one of the serious problems for HYCOL gasifiers, as
Figure 6. Temperature profiles (in Kelvin) at a load ratio of 50% and a load ratio of 150%, where the load ratio is defined as (load)/(basic load).
Coal Gasification in an Entrained Flow Gasifier. II
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ment. Moreover, according to this research, in addition to staging of the gasification and the O2/coal ratio, the O2 partition between the lower and upper burners is also an important factor, because it influences the temperature distribution in the gasifier, although its influence on the carbon conversion or cold gas efficiency is not so significant. Conclusions
Figure 7. Effect of load ratio on the residence times of particles, where the load ratio is defined as (load)/(basic load).
Figure 8. Carbon conversion and cold gas efficiency versus O2/coal ratio: comparison between experimental data and predictions. Symbols represent experimental data, whereas curves represent predicted values.
encountered during the experiments on the 50 ton/day pilot-scale HYCOL gasifier in Japan. This type of information can be conveniently derived from simulations, but is almost impossible to obtain from experi-
The simulator that we developed has satisfactory sensitivity, with respect to not only the change in coal properties, but also the change in operating conditions. The O2 partition between the lower and upper burners significantly influenced the temperature distribution in a gasifier. Staged gasification not only enhanced carbon conversion and cold gas efficiency, but also led to favorable temperature distribution that suppresses ash slagging in the gasifier. The carbon conversion increased as the O2/coal ratio increased, whereas the cold gas efficiency had a maximum as the O2/coal ratio increased. The carbon conversion and cold gas efficiency decreased as the load ratio of a gasifier increased, but with limited magnitude. Satisfactory agreement between predictions and pilot-scale experimental results was obtained. According to this work, O2 partition between the lower and upper burners and the O2/coal ratio must be chosen with care. The results obtained in this research may be useful information for the designing and operation of a gasifier. Acknowledgment. The authors would like to thank NEDO/CCUJ for financial support of this work under BRAIN-C program. EF030163J
