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 comminutionExperiments on fuels comminution CFBC simulation on industrial-scaleCFBC simulation on industrial-scale CFBC simulation on laboratory-scaleCFBC simulation on laboratory-scale ConclusionsConclusions

Research Objectives Study the comminution of local coal and biomass.Study the comminution of local coal and biomass. Model and simulate a circulating fluidized bed combustor by including the condition of the comminution effect.Model and simulate a circulating fluidized bed combustor by including the condition of the comminution effect.

Experiments on Fuels Comminution

Experimental procedures Blank studyBlank study Attrition studyAttrition study Primary fragmentation studyPrimary fragmentation study Secondary fragmentation studySecondary fragmentation study

ApparatusApparatus Figure 1 Apparatus

Table 1 Operating conditions of the CFB reactor for communition test communition test Table 1 Operating conditions of the CFB reactor for communition test communition test 29

Figure 2The PSD of sand from blank study at 850 o C, 1 atm Figure 2 The PSD of sand from blank study at 850 o C, 1 atm that analyzed by particle size laser analyzer. that analyzed by particle size laser analyzer. Figure 2The PSD of sand from blank study at 850 o C, 1 atm Figure 2 The PSD of sand from blank study at 850 o C, 1 atm that analyzed by particle size laser analyzer. that analyzed by particle size laser analyzer. Blank Study

Attrition Study Figure 3 Mixed particle between coal and sand after attrition study at the ambient environment by CCD camera. study at the ambient environment by CCD camera. Figure 3 Mixed particle between coal and sand after attrition study at the ambient environment by CCD camera. study at the ambient environment by CCD camera.

(a) (b) Figure 4 PSD from Image Pro Plus: (a) raw material, (b) attrition particles (a) raw material, (b) attrition particles Figure 4 PSD from Image Pro Plus: (a) raw material, (b) attrition particles (a) raw material, (b) attrition particles

Figure 5 The PSD of mixed particles from attrition study at ambient environment that analyzed by particle size ambient environment that analyzed by particle size laser analyzer compare with blank study at 850 o C, laser analyzer compare with blank study at 850 o C, 1 atm. 1 atm. Figure 5 The PSD of mixed particles from attrition study at ambient environment that analyzed by particle size ambient environment that analyzed by particle size laser analyzer compare with blank study at 850 o C, laser analyzer compare with blank study at 850 o C, 1 atm. 1 atm.

Primary Fragmentation Study Figure 6 Mixed particle between coal and sand after primary fragmentation study at 850 o C, 1 atm with N 2 as the fragmentation study at 850 o C, 1 atm with N 2 as the fluidizing gas by CCD camera. fluidizing gas by CCD camera. Figure 6 Mixed particle between coal and sand after primary fragmentation study at 850 o C, 1 atm with N 2 as the fragmentation study at 850 o C, 1 atm with N 2 as the fluidizing gas by CCD camera. fluidizing gas by CCD camera.

Figure 7 PSD from Image Pro Plus of primary fragmentation particles. particles. Figure 7 PSD from Image Pro Plus of primary fragmentation particles. particles.

Figure 8 Compare the cumulative fraction between the experiment of primary fragmentation at 850 o C, 1 atm with N 2 as the of primary fragmentation at 850 o C, 1 atm with N 2 as the fluidizing gas and the model prediction for large particles. fluidizing gas and the model prediction for large particles. Figure 8 Compare the cumulative fraction between the experiment of primary fragmentation at 850 o C, 1 atm with N 2 as the of primary fragmentation at 850 o C, 1 atm with N 2 as the fluidizing gas and the model prediction for large particles. fluidizing gas and the model prediction for large particles. Mean diameter 2.12E-3

Figure 9 The PSD of mixed particles from primary fragmentation study at 850 o C, 1 atm with N 2 as the fluidizing gas that study at 850 o C, 1 atm with N 2 as the fluidizing gas that analyzed by particle size laser analyzer compare with analyzed by particle size laser analyzer compare with blank study at 850 o C, 1 atm. blank study at 850 o C, 1 atm. Figure 9 The PSD of mixed particles from primary fragmentation study at 850 o C, 1 atm with N 2 as the fluidizing gas that study at 850 o C, 1 atm with N 2 as the fluidizing gas that analyzed by particle size laser analyzer compare with analyzed by particle size laser analyzer compare with blank study at 850 o C, 1 atm. blank study at 850 o C, 1 atm.

Figure 10Compare the cumulative fraction between the Figure 10 Compare the cumulative fraction between the experiment of primary fragmentation at 850 o C, 1 atm experiment of primary fragmentation at 850 o C, 1 atm with N 2 as the fluidizing gas and the model prediction with N 2 as the fluidizing gas and the model prediction for small particles. for small particles. Figure 10Compare the cumulative fraction between the Figure 10 Compare the cumulative fraction between the experiment of primary fragmentation at 850 o C, 1 atm experiment of primary fragmentation at 850 o C, 1 atm with N 2 as the fluidizing gas and the model prediction with N 2 as the fluidizing gas and the model prediction for small particles. for small particles. Mean diameter 627

Secondary Fragmentation Study Figure 11 Mixed particle between coal and sand after secondary fragmentation study at 850 o C, 1 atm with air as the fragmentation study at 850 o C, 1 atm with air as the fluidizing gas. fluidizing gas. Figure 11 Mixed particle between coal and sand after secondary fragmentation study at 850 o C, 1 atm with air as the fragmentation study at 850 o C, 1 atm with air as the fluidizing gas. fluidizing gas.

Figure 12 The PSD of mixed particles from secondary fragmentation study at 850 o C, 1 atm with air as the fragmentation study at 850 o C, 1 atm with air as the fluidizing gas that analyzed by particle size laser fluidizing gas that analyzed by particle size laser analyzer compare with blank study at 850 o C, 1 atm. analyzer compare with blank study at 850 o C, 1 atm. Figure 12 The PSD of mixed particles from secondary fragmentation study at 850 o C, 1 atm with air as the fragmentation study at 850 o C, 1 atm with air as the fluidizing gas that analyzed by particle size laser fluidizing gas that analyzed by particle size laser analyzer compare with blank study at 850 o C, 1 atm. analyzer compare with blank study at 850 o C, 1 atm.

Figure 13 Cumulative fraction of secondary fragmentation at 850 o C, 1 atm with air as the fluidizing gas. 850 o C, 1 atm with air as the fluidizing gas. Figure 13 Cumulative fraction of secondary fragmentation at 850 o C, 1 atm with air as the fluidizing gas. 850 o C, 1 atm with air as the fluidizing gas. Unburnt carbon Ash

Figure 14 Compare the cumulative fraction between the experiment of secondary fragmentation at 850 o C, 1 atm experiment of secondary fragmentation at 850 o C, 1 atm with air as the fluidizing gas and the model prediction for with air as the fluidizing gas and the model prediction for ash particles. ash particles. Figure 14 Compare the cumulative fraction between the experiment of secondary fragmentation at 850 o C, 1 atm experiment of secondary fragmentation at 850 o C, 1 atm with air as the fluidizing gas and the model prediction for with air as the fluidizing gas and the model prediction for ash particles. ash particles. Mean diameter 25

Figure 15 Compare the cumulative fraction between the experiment of secondary fragmentation at 850 o C, 1 atm experiment of secondary fragmentation at 850 o C, 1 atm with air as the fluidizing gas and the model prediction for with air as the fluidizing gas and the model prediction for unburnt particles. unburnt particles. Figure 15 Compare the cumulative fraction between the experiment of secondary fragmentation at 850 o C, 1 atm experiment of secondary fragmentation at 850 o C, 1 atm with air as the fluidizing gas and the model prediction for with air as the fluidizing gas and the model prediction for unburnt particles. unburnt particles. Mean diameter 295

Biomass Study (a) (b) Figure 16 The PSD of bagasse-sand particles at 850 o C, 1 atm by particle size laser analyzer: (a) primary fragmentation, particle size laser analyzer: (a) primary fragmentation, N 2 as the fluidizing gas (b) secondary fragmentation, N 2 as the fluidizing gas (b) secondary fragmentation, air as the fluidizing gas. air as the fluidizing gas. Figure 16 The PSD of bagasse-sand particles at 850 o C, 1 atm by particle size laser analyzer: (a) primary fragmentation, particle size laser analyzer: (a) primary fragmentation, N 2 as the fluidizing gas (b) secondary fragmentation, N 2 as the fluidizing gas (b) secondary fragmentation, air as the fluidizing gas. air as the fluidizing gas.

CFBC Simulation on Industrial-scale

21.84 m m m 1.5 m Primary air Secondary air Tertiary air m Figure17 Dimension of combustor. Dimension of CFBC

Assumptions of the reaction model The fuel, limestone, and primary air were fed at the bottom of the CFBC with a uniform temperature.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 m 2 and the height of m. In the proposed model, the secondary and tertiary air was fed into the combustor at the specified height.The simulated combustor was a rectangular column with the surface area of m 2 and the height of 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.The combustion of volatile matters occurred instantaneously at the bottom of the combustor. Char combustion occurred slowly after volatile matters were combusted.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.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.The attrition of the char particles was neglected. All steps of the reactions were calculated with an isothermal at 850 O C.All steps of the reactions were calculated with an isothermal at 850 O C.

LOWER REGION UPPER REGION 1 st Interval 2 nd Interval 3 rd Interval Figure 18 Simulation diagram for the CFBC

Simulation Procedures Simulation Procedures [Sotudeh-Gharebaagh 1998] Devolatilization and volatilize combustionDevolatilization and volatilize combustion Char combustionChar combustion NO x formationNO x formation SO 2 absorptionSO 2 absorption

Results and discussion The model was used to simulate the operation of a CFBC that produced 110 tons/hr of steam at 510 o C 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 20 Rates of the combustion of lignite in mixed fuels for each region in the CFBC: (a) lignite&bagasse (b) lignite&bark (c) lignite&sludge in the CFBC: (a) lignite&bagasse (b) lignite&bark (c) lignite&sludge

Figure 21 Rates of the combustion of biomass in mixed fuels for each region in the CFBC: (a) lignite&bagasse (b) lignite&bark (c) lignite&sludge in the CFBC: (a) lignite&bagasse (b) lignite&bark (c) lignite&sludge

Figure 22 The composition of flue gas for different kind of mixed fuel: (a) lignite&bagasse (b) lignite&bark (c) lignite&sludge (a) lignite&bagasse (b) lignite&bark (c) lignite&sludge

CFBC Simulation on Laboratory-scale

Assumptions of the reaction model The fuel, limestone, and primary air were fed at the bottom of the CFBC with a uniform temperature.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.The combustion of volatile matters occurred instantaneously at the bottom of the combustor. Char combustion occurred slowly after volatile matters were combusted.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.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 O C.All steps of the reactions were calculated with an isothermal at 850 O C.

Figure 23 Simulation diagram for the laboratory scale CFBC.

Weibull distribution for the primary fragmentation Large particles Small particles

Results and discussion In the simulation, coal and air was fed at 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.

Figure 24Particle size distribution of initial particle: (a) input to shrinking Figure 24 Particle size distribution of initial particle: (a) input to shrinking core model simulation, (b) input to shrinking core model with core model simulation, (b) input to shrinking core model with primary fragmentation model. primary fragmentation model. (a) (b)

(a) (b) Figure 25 Particle size distribution after devolatilization process at 850 o C, 1 atm: (a) no adding primary fragmentation model, (b) adding 1 atm: (a) no adding primary fragmentation model, (b) adding primary fragmentation model. primary fragmentation model.

Figure 26 Particle size distribution after combustion in lower region at 850 o C, 1 atm :(a) no adding primary fragmentation (b) adding 850 o C, 1 atm :(a) no adding primary fragmentation (b) adding primary fragmentation model. primary fragmentation model. (a) (b)

ConclusionsConclusions 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  m 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, N 2 O, and SO 2. 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.

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. Overall Conclusions

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