This study is supported by the Economic Development Board (EDB) of Singapore under grant number R-261-501-003-414 through the Minerals, Metals and Materials Technology Center (M3TC) of the National University of Singapore (NUS)
Coal gasification is an effective way to reduce the emission of CO2 and alleviate the global warming. The gasification can convert various carbon-based feed stocks to clean synthetic gases, mainly the mixture of H2 and CO, then they can be further used to produce other chemicals, or act as fuel to produce the electricity through Integrated Gasification Combined Cycle (IGCC). The gasification is quite complex including many processes, such as the heating, pyrolysis, gasification, combustion, tar reforming etc, and in the traditional gasifiers all the processes happen in the same reactor. In order to improve the gasification efficiency and reduce the emission of pollutions, we proposed triple-bed combined circulating fluidized bed to decouple different processes, as seen in the figure below, so the processes can manipulated individually and further synthesized according to their different characteristics.
1. Triple-bed combined circulating fluidized bed
Cheng Y. P., Lau D. Y. J., Guan G. Q., Fushimi C., Tsutsumi A., Wang C. H. Experimental and numerical studies on the electrostatics generation and transport in the triple-bed circulating fluidized bed. Industrial & Engineering Chemistry Research, 51: 14258-14267, 2012.
Guan G. Q., Fushimi C., Ishizuka M., Nakamura Y., Tsutsumi A. Mstusda S., Suzuki Y., Hatano H., Cheng Y.P., Lim E. W. C., Wang C. H. Flow behaviors in the downer of a large-scale triple-bed combined circulating fluidized system with high solids mass fluxes. Chemical Engineering Science, 66 (18): 4212-4220, 2011.
In the triple-bed combined circulating fluidized bed, coal pyrolysis, gasification and combustion are separated and they will happen in the downer, bubbling fluidized and riser respectively. Coal is fed into the downer through the nozzles at the top of downer, after mixing with the hot circulating silica sand, coal will gain the heat and start the pyrolysis, through the gas-solid separator at the bottom of downer, the produced gases and tar are separated from the char, then the char enters the bubbling fluidized bed to be gasified with steam. The unreacted char flows downward to the bottom of riser, where it will be combusted with air and generate heat. The heat then will be transferred by inert silica sand to the downer and bubbling fluidized bed for coal pyrolysis and gasification.
Figure 1 Triple-bed combined circulating fluidized bed
2. Heat transfer performance in downer for coal pyrolysis
Cheng Y.P., Guan G.Q., Lim E. W. C., Fushimi C., Ishizuka M., Wang C. H., Tsutsumi A. Numerical simulations and experiments on heat transfer around a probe in the downer reactor. Powder Technology, 2012, 235: 359-367, 2013.
The heat transfer performance of a downer reactor has great significance for coal gasification processes due to the very short residence time. In this study both experimental and numerical works were carried out to study heat transfer around a heating probe in a downer. Both experimental results and numerical results revealed that average heat transfer coefficients decreased with increasing superficial air velocity or decreasing solids mass flux, which could be attributed to decreasing solids holdup, as particle-particle convection was dominant in the heat transfer mechanism for the current cases. Numerical simulations also revealed that heat transfer may deteriorate with increasing particle size. Finally it was found that the heat transfer performance with constant temperature boundary condition around the probe was much better than that with constant heat flux boundary condition. These results may help us better understand heat transfer in downers for improving the design of downers with higher efficiencies.
Figure 2 Arrangement of heat transfer probe in the downer
Figure 3 Comparison of experimental and numerical results
3. Mixing of coal and sand in the downer
Cheng Y.P., Lim E.W.C., Wang C.H., Guan G.Q., Nakamura Y., Ishizuka M., Fushimi C. and Tsutsumi A., Studies of solid-solid mixing behaviors in a downer reactor, AIChE annual meeting, Minneapolis, Minnesota, USA, 16-21, Oct. 2011.
In the downer reactor, the silica sand (red) flows from the top and coal particles (black) are injected from the nozzles. As the residence time of particles in the downer is quite short, the mixing between coal and silica sand is very critical to achieve high pyrolysis efficiency. In our study, the Discrete Element Method coupled with Computational Fluid Dynamics is adopted to simulate the motions of silica sand and coal particles, and their mixing is characterized based on their motions, then the optimal injection nozzle arrangement and operating conditions are obtained.
Figure 4 Coal (black) and silica sand (red) mixing in the downer
4. Electrostatic characteristics in large-scale triple-bed combined circulating fluidized bed for coal gasification.
Cheng Y. P., Lim E. W. C., Wang C. H., Guan G. Q., Fushimi C., Ishizuka M., Tsutsumi A. Electrostatic characteristics in a large-scale triple-bed circulating fluidized system for coal gasification. Chemical Engineering Science, 75: 435-444, 2012.
Electrostatics charge generation by triboelectrification has significant implications for the proper design and operation of a circulating fluidized bed. In this study, electrostatics in the fully developed regions of both the riser and downer of a large-scale triple-bed combined circulating fluidized bed was characterized in terms of the equivalent currents over the cross section of the developed region. The average equivalent currents and solids holdup were measured under different superficial velocities in the riser, downer and gas-sealing bed. It was found that in the fast fluidization regime in the riser, the negative equivalent currents were comparable with the positive equivalent currents due to the typical core-annulus flow pattern. With increasing superficial velocities in the riser or in the gas-sealing bed, the flow pattern would approach dilute phase transport or dense suspension upflow regime. Thus, the positive equivalent currents became dominant because the backflow of sand particles were greatly suppressed. Some dominant frequencies for the equivalent currents in the riser were almost identical regardless of the magnitude of superficial velocities in the riser and the gas-sealing bed, indicating that they were determined by the inherent characteristics of electrostatics and/or signal noise, and were not affected by gas¨Csolids flow behaviors. The frequencies in the downer were focused on the low value range, and the dominant frequencies were nearly zero. Both the solids mass flux and solids holdup had significant influence on the average equivalent currents. With increasing superficial velocities in the gas-sealing bed, the average equivalent currents first increased and then approached a constant. This led to the same trend in variation of solids mass flux, as well as solids holdup. With increasing air superficial velocities in the riser, the average equivalent currents in the riser increased, while the average equivalent currents decreased in the downer with increasing air superficial velocities in the downer.
Figure 5 Large-scale triple-bed combined circulating fluidized bed for coal gasification
Figure 6 Measurement with Modular Parametric Current Transfer (MPCT)
5. Electrostatic sensors for velocity measurement
- B. Zhang, Y. P. Cheng, C. H. Wang, C. Wang, W. Q. Yang Investigation on Hydrodynamics of Triple-Bed Combined Circulating Fluidized Bed using Electrostatic Sensor and Electrical Capacitance Tomography, 9th European Congress of Chemical Engineering, World Forum, The Hagen, Netherlands, April 21-25, 2013.
In the circulating fluidized bed, due to the frequent collisions and frictions between particle-particle and particle-wall, electrostatic charge is generated on the solid particles and walls. When charged particles move near the electrostatic electrodes, the charges are induced on the electrodes. By cross-correlating the signals on the induced charges from upstream and downstream ring-shape electrodes, the average solids velocity can be obtained. Based on this principle, we have proposed the electrostatic sensors, including ring-shape electrodes (used to measure cross-sectional average solid velocity) and arc-shape electrodes (used to measure local solid velocity). The measured solid velocities are also compared with those with Electrical Capacitance Tomography (ECT). The combined use of ECT sensors and electrostatic sensors may extend the measurement range of solids velocity.
Figure 7 Solid velocity in the riser (250 mm above the entrance) with ECT sensors and electrostatic sensors