Jeong-Hoo Choi

Oxidation and Reduction Characteristics of Oxygen Carrier Particles and Reaction Kinetics by Unreacted Core Model

The reaction kinetics of the oxygen carrier particles, which are used as bed material for a fluidized bed chemical looping combustor (CLC), has been studied experimentally by a conventional thermal gravimetrical analysis technique. The weight percent of nickel and nickel oxide in oxygen carrier particles and reaction temperature were considered as experimental variables. After oxidation reaction, the pure nickel particle was sintered and unsuitable to use as fluidizing particles. The oxidation reaction rate increased with increasing weight percent of nickel in oxygen carrier particles and reaction temperature. The rate of reduction shows maximum point with weight percent of nickel oxide (57.8%) and reaction temperature (750 or 800 °C) increased. In this work, the reaction between air and Ni/ bentonite particle was described by a special case of unreacted core model in which the global reaction rate is controlled by product layer diffusion resistance. However, the reaction between CH4 and NiO/bentonite particle was described by unreacted core model in which the global reaction rate is controlled by chemical reaction resistance. The temperature dependence of the effective diffusivity of oxidation reaction and reaction rate constant of reduction reaction could be calculated from experimental data and fitted to the Arrhenius equation.

The effect of fine particles on elutriation of coarse particles in a gas fluidized bed

The effect of fine particles on the elutriation rate of the coarse particles at the gas exit was investigated in a gas-fluidized bed (i.d.: 0.1 m, height: 1.97 m) with sand as bed material. The elutriation rate of the coarse particles increased with the gas velocity and the proportion of fine particles in the bed. It was found to increase nearly proportionally with the upward momentum of the fine particles calculated on bases of unit mass and composition of bed particles at a specified gas velocity. The effect of the fine particles on the elutriation of the coarse particles decreased as the gas velocity increased. The elutriation of the fine particles was not affected by the size distribution of bed particles. An improved correlation to determine the elutriation rate of the coarse particles has been proposed for sand particles.

The effect of temperature on particle entrainment rate in a gas fluidized bed

Abstract The qualitative effect of temperature on the particle entrainment rate has been measured in a gas fluidized bed (i.d. 0.1 m, height 1.97 m) equipped with an electric heater, using sand as the bed material. The mean particle size (0.091–0.363 mm), the gas velocity (0.65–2.3 m/s) and the bed temperature (20–600°C) were considered as experimental variables. As the temperature was increased, the particle entrainment rate was observed to decrease initially. However, above a certain temperature it began to increase for gas velocities between 0.8 and 1.4 m/s. The variation of the particle entrainment rate with temperature was similar to that of the particle size for which the terminal velocity is equal to the gas velocity.

Effect of secondary gas injection on the particle entrainment rate in a gas fluidized bed

Abstract The effect of secondary gas injection on the particle entrainment rate was measured in a cold model fluidized bed (i.d. 0.1 m, height 2.25 m) and discussed. Sand particles below 0.425 mm in screen size were used as bed materials. The particle size (0.128–0.363 mm), the overall superficial gas velocity (0.78–2.76 m/s), the secondary gas fraction (0–0.5), and the static bed height (0.1–0.3 m) were considered as experimental variables. The particle entrainment rate decreased with an increase of the secondary gas fraction. The injection level of the secondary gas was shown to have an important influence on its effect. The effect of the secondary gas was appreciable for over-bed injection; the effect was reduced, however, when the injection level was placed in the splash zone or in the dense phase by means of increasing the bed height.

Axial voidage profile in a cold model circulating fluidized bed

The axial voidage profile was measured in a cold model circulating fluidized bed (0.38 m in diameter and 9.1 m in height) of sand particles as bed materials. Voidage in the riser column increases along the height above the distributor plate with increasing the gas velocity. However, it decreases with an increase in solid circulation rate in the bed. Model correlations to predict the solid circulation rale and the axial voidage profile in the bed are proposed.

Simulation of a bubbling fluidized bed process for capturing CO2 from flue gas

We simulated a bubbling bed process capturing CO2 from flue gas. It applied for a laboratory scale process to investigate effects of operating parameters on capture efficiency. The adsorber temperature had a stronger effect than the regenerator temperature. The effect of regenerator temperature was minor for high adsorber temperature. The effect of regenerator temperature decreased to level off for the temperature >250 °C. The capture efficiency was rather dominated by the adsorption reaction than the regeneration reaction. The effect of gas velocity was as appreciable as that of adsorber temperature. The capture efficiency increased with the solids circulation rate since it was ruled by the molar ratio of K to CO2 for solids circulation smaller than the minimum required one (Gs, min). However, it leveled off for solids circulation rate >Gs, min. As the ratio of adsorber solids inventory to the total solids inventory (xw1) increased, the capture efficiency increased until xw1=0.705, but decreased for xw1>0.705 because the regeneration time decreased too small. It revealed that the regeneration reaction was faster than the adsorption reaction. Increase of total solids inventory is a good way to get further increase in capture efficiency.

Effect of temperature on slug properties in a gas fluidized bed

Abstract In order to understand the effect of temperature on slug properties, the onset velocity of slugging, slug rising velocity and slug frequency have been measured by a differential pressure method in an electrically heated gas fluidized bed of 0.1 m i.d. and height of 1.97 m and, with respect to variations in gas velocity ( U mf ∼0.15 m/s) and temperature (25–400 °C). Air was used as fluidizing gas and fluid catalytic cracking (FCC) catalyst particle (Geldarts group-A particle, d p =0.071 mm, ρ p =1600 kg/m 3 ) as bed material. The minimum slugging velocity was found to increase a little with bed temperature. The qualitative change in minimum slugging velocity was found to agree with the inverse of minimum fluidizing velocity as temperature was varied. As the bed temperature increased, slug frequency was found to decrease a little, whereas slug rising velocity increased. A correlation between slug rising velocity and bed temperature was proposed.

Solids circulation rate and static bed height in a riser of a circulating fluidized bed

Solids circulation rate and static bed height in the riser of a circulating fluidized bed (CFB) process, which consisted of a riser and two bubbling-beds, were investigated and discussed at ambient temperature and pressure. Three kinds of powder (FCC catalyst, glass bead, plastic powder) were used as bed materials. The static bed height in the riser increased with the solids circulation rate. However, it decreased with an increase of gas velocity. The effect of gas velocity diminished as the gas velocity increased. The riser static bed height could be used to estimate the solids circulation rate in reasonable accuracy. A correlation on static bed height in the riser, relating to the solids circulation rate, was proposed for the present experimental ranges.

A model on chemical looping combustion of methane in a bubbling fluidized-bed process

We developed a mathematical model to discuss the performance of chemical looping combustion (CLC) of methane in continuous bubbling fluidized-beds. The model considers the particle population balance, oxidation and reduction rate of particles in fluidized beds. It also considers utilization efficiency of oxygen carrier (OC) particles, residence time of particles in each reactor, and particle size in reaction rate. The model was applied for a bubbling coreannulus fluidized-bed process. The core bed was the fuel reactor (0.08 m-i.d., 2.1 m-height) and the annulus bed was the air reactor (0.089 m-i.d., 0.15 m-o.d., 1.6 m-height). The process employed a type of Ni-based OC particles. The present model agrees reasonably well with the combustion efficiency measured in the process. Simulation was performed to investigate the effects of some variables for the process. The present model revealed that the range of circulation rate of OC particles for achieving complete combustion determined the operating range of the CLC system. The minimum circulation rate of OC particles for complete combustion decreased in the considered operating range as temperature or bed mass increased in the fuel reactor. A large mass of the fuel bed was necessary to obtain complete combustion at low fuel reactor temperature. The fresh feed rate of OC particles for steady state operation increased in complete combustion condition as temperature or static bed height or gas velocity increased.

Predicting the fast transition conditions by the correlation of particle entrainment rate

A model for predicting the fast transition condition in a riser of a circulating fluidized bed was proposed using the correlation of particle entrainment rate. The saturation carrying capacity of Bai and Kato could be regarded as the particle entrainment rate at the fast transition condition. The correlation of Choi et al. on particle entrainment rate could be used as a tool to predict the fast transition condition. The effect of interparticle forces seemed to be negligible at the fast transition condition. The model was in fair agreement with the measured values at the fast transition condition.