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Modeling and Simulation of Size Reduction of Fuels in Circulating Fluidized Bed Combustor by Considering Attrition and Fragmentation. By Natthapong Ngampradit, Ph.D. 14 Dec 2006. Outline. Research objectives Experiments on fuels comminution CFBC simulation on industrial-scale
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Modeling and Simulation of Size Reduction of Fuelsin Circulating Fluidized Bed Combustor by Considering Attrition and Fragmentation By Natthapong Ngampradit, Ph.D. 14 Dec 2006
Outline Research objectives • Experiments on fuels comminution • CFBC simulation on industrial-scale • CFBC simulation on laboratory-scale • Conclusions
Research Objectives • Study the comminution of local coal and biomass. • Model and simulate a circulating fluidized bed combustor by including the condition of the comminution effect.
Experimental procedures • Blank study • Attrition study • Primary fragmentation study • Secondary fragmentation study
Apparatus Figure 1Apparatus
Table 1Operating conditions of the CFB reactor for communition test 29
BlankStudy Figure 2The PSD of sand from blank study at 850 oC, 1 atm that analyzed by particle size laser analyzer.
AttritionStudy Figure 3Mixed particle between coal and sand after attrition study at the ambient environment by CCD camera.
(b) (a) Figure 4PSD from Image Pro Plus: (a) raw material, (b) attrition particles
Figure 5The PSD of mixed particles from attrition study at ambient environment that analyzed by particle size laser analyzer compare with blank study at 850 oC, 1 atm.
Primary FragmentationStudy Figure 6Mixed particle between coal and sand after primary fragmentation study at 850 oC, 1 atm with N2 as the fluidizing gas by CCD camera.
Figure 7PSD from Image Pro Plus of primary fragmentation particles.
Mean diameter 2.12E-3 Figure 8Compare the cumulative fraction between the experiment of primary fragmentation at 850 oC, 1 atm with N2 as the fluidizing gas and the model prediction for large particles.
Figure 9The PSD of mixed particles from primary fragmentation study at 850 oC, 1 atm with N2 as the fluidizing gas that analyzed by particle size laser analyzer compare with blank study at 850 oC, 1 atm.
Mean diameter 627 Figure 10Compare the cumulative fraction between the experiment of primary fragmentation at 850 oC, 1 atm with N2 as the fluidizing gas and the model prediction for small particles.
Secondary FragmentationStudy Figure 11Mixed particle between coal and sand after secondary fragmentation study at 850 oC, 1 atm with air as the fluidizing gas.
Figure 12The PSD of mixed particles from secondary fragmentation study at 850 oC, 1 atm with air as the fluidizing gas that analyzed by particle size laser analyzer compare with blank study at 850 oC, 1 atm.
Unburnt carbon Ash Figure 13Cumulative fraction of secondary fragmentation at 850 oC, 1 atm with air as the fluidizing gas.
Mean diameter 25 Figure 14Compare the cumulative fraction between the experiment of secondary fragmentation at 850 oC, 1 atm with air as the fluidizing gas and the model prediction for ash particles.
Mean diameter 295 Figure 15Compare the cumulative fraction between the experiment of secondary fragmentation at 850 oC, 1 atm with air as the fluidizing gas and the model prediction for unburnt particles.
BiomassStudy (a) (b) Figure 16The PSD of bagasse-sand particles at 850 oC, 1 atm by particle size laser analyzer: (a) primary fragmentation, N2 as the fluidizing gas (b) secondary fragmentation, air as the fluidizing gas.
21.84 m Tertiary air 1.5 m 6.026 m Secondary air 1.703 m Primary air 6.026 m Dimension of CFBC Figure17Dimension of combustor.
Assumptions of the reaction model • The fuel, limestone, and primary air were fed at the bottom of the CFBC with a uniform temperature. • The simulated combustor was a rectangular column with the surface area of 36.31 m2 and the height of 21.84 m. In the proposed model, the secondary and tertiary air was fed into the combustor at the specified height. • The combustion of volatile matters occurred instantaneously at the bottom of the combustor. • Char combustion occurred slowly after volatile matters were combusted. • Gas and fuel particle temperatures were equal to the bed temperatures varying with respect to the height of the riser. • The attrition of the char particles was neglected. • All steps of the reactions were calculated with an isothermal at 850 OC.
1st Interval 2nd Interval 3rd Interval UPPER REGION LOWER REGION Figure 18Simulation diagram for the CFBC
Simulation Procedures[Sotudeh-Gharebaagh 1998] • Devolatilization and volatilize combustion • Char combustion • NOx formation • SO2 absorption
Results and discussion The model was used to simulate the operation of a CFBC that produced 110 tons/hr of steam at 510 oC and 110 barg. The fuels to be considered were both of single fuels and mixed fuels. In case of a single fuel, 4 kg/s of lignite were fed into the combustor. The other case, the mixed fuels between lignite and biomass were considered. Each simulation of the mixtures was decreased the lignite flow rate by 10 %. The flow rate of biomass was increased for keeping the constant of amount of carbon.
Figure 20Rates of the combustion of lignite in mixed fuels for each region in the CFBC: (a) lignite&bagasse (b) lignite&bark (c) lignite&sludge
Figure 21Rates of the combustion of biomass in mixed fuels for each region in the CFBC: (a) lignite&bagasse (b) lignite&bark (c) lignite&sludge
Figure 22The composition of flue gas for different kind of mixed fuel: (a) lignite&bagasse (b) lignite&bark (c) lignite&sludge
Assumptions of the reaction model • The fuel, limestone, and primary air were fed at the bottom of the CFBC with a uniform temperature. • The combustion of volatile matters occurred instantaneously at the bottom of the combustor. • Char combustion occurred slowly after volatile matters were combusted. • Gas and fuel particle temperatures were equal to the bed temperatures varying with respect to the height of the riser. • All steps of the reactions were calculated with an isothermal at 850 OC.
Weibull distribution for the primary fragmentation Large particles Small particles
Results and discussion In the simulation, coal and air was fed at 0.015 g∙s-1 and 7 l∙min-1. The simulations were divided in two cases. The first case, the PSD was calculated only by the shrinking core model subroutine. The second one, the primary fragmentation model that fitted by Weibull distribution was added in the lower region to predict the coal comminution from the devolatilization process.
(a) (b) Figure 24Particle size distribution of initial particle: (a) input to shrinking core model simulation, (b) input to shrinking core model with primary fragmentation model.
(a) (b) Figure 25Particle size distribution after devolatilization process at 850 oC, 1 atm: (a) no adding primary fragmentation model, (b) adding primary fragmentation model.
0.45 0.25 (a) (b) Figure 26Particle size distribution after combustion in lower region at 850 oC, 1 atm :(a) no adding primary fragmentation (b) adding primary fragmentation model.
Conclusions The experiments on the fuels comminution Primary fragmentation study The models to predict the particle size distribution were divided into two models as showed in the following equations. For the small particles with size between 500-750 mm For the large particles with size between 1-3 mm
Secondary fragmentation study The models to predict the particle size distribution for the coal particles after combustion were divided into two models as showed in the following equations: For the fine particles For the coarse particles
Industrial scale CFBC simulation This section was proposed a model for simulating a CFBC using single or mixed fuels. The shrinking core model was included in the simulation to calculate the size distribution and weight fractions in each region of the riser. The modification will reflect the phenomena in the riser better. Moreover, the detail of emission models were added in the simulation to predict the formation of NO, N2O, and SO2. For different biomass fractions in the fuel, the simulation output will demonstrate the trend of gas emission, which can be used for environment protection consideration.
Laboratory-scale CFBC simulation The simulation in this section emphasized on the particle size distribution in the riser of the CFBC. Two case studies were simulated. The first case, only shrinking core model was added to predict the PSD along the riser. The second case, the Weibull distribution was added at the bottom of riser to predict the PSD after the devolatilization process. It was found that the sizes of particles were reduced along the riser. The second case could be predicted the fine particles better than the first case. This was due to only the shrinking core model could not eliminate the large particle in the system. The original size of particles still remains at the top of riser. However, the result of the second case simulation was not coincided with the experiment result because of the difference in operating modes.
Overall Conclusions This research was studied the comminution of Thailand coal. The CCD camera and particle size laser analyzer were used to measure the size of particles because these method disturb the fragmented particles less than the sieve analysis method. The Weibull distribution was used to predict the particle size distribution for the fragmented particles. Moreover, in the simulation part, the PSD was predicted along the riser of the CFBC.
