Gyoung-Tae Jin

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.

Demonstration of inherent CO2 separation and no NOx emission in a 50kW chemical-looping combustor: Continuous reduction and oxidation experiment

Publisher Summary The chapter explains a study that seeks to display a chemical-looping combustion system by a continuous reduction and oxidation experiment in a 50kWth chemical-looping combustor with high CO2 selectivity, no side reaction (carbon deposition and/or hydrogen generation), inherent CO2 separation, and no NOx emission. In this study, NiO/bentonite acts as an oxygen carrier particle in the bed material of the chemical-looping combustion system and CH4 and air are used as reacting gases for reduction and oxidation, respectively. NO, NO2, and N2O concentrations are measured to check whether NOx is formed during oxidation and measured CO2, CO, CH4, H2 concentration to show inherent CO2 separation and no side reaction during reduction. It takes more than 3.5 hour to finish the process. Pressure drop profiles in a system loop are maintained steadily throughout the 3.5 hour run and solid circulation between an oxidizer and a reducer remains smooth and stable. By analysis of gases in the exit stream from the system, the study concluded that inherent CO2 separation and NOx-free combustion are possible in the chemical-looping combustion system.

Effect of temperature on reduction reactivity of oxygen carrier particles in a fixed bed chemical-looping combustor

In a chemical-looping combustor (CLC), gaseous fuel is oxidized by metal oxide particle, e.g. oxygen carrier, in a reduction reactor (combustor), and the greenhouse gas CO2 is separated from the exhaust gases during the combustion. In this study, NiO/bentonite particle was examined on the basis of reduction reactivity, carbon deposition during reduction, and NOx formation during oxidation. Reactivity data for NiO/bentonite particle with methane and air were presented and discussed. During the reduction period, most of the CH4 are converted to CO2 with small formation of CO. Reduction reactivity (duration of reduction) of the NiO/bentonite particle increased with temperature, but at higher temperature, it is somewhat decreased. The NiO/bentonite particle tested showed no agglomeration or breakage up to 900 ‡C, but at 1,000 ‡C, sintering took place and lumps of particles were formed. Solid carbon was deposited on the oxygen carrier during high conversion region of reduction, i.e., during the end of reduction. It was found that the appropriate temperature for the NiO/bentonite particle is 900 ‡C for carbon deposition, reaction rate, and duration of reduction. We observed experimentally that NO, NO2, and N2O gases are not generated during oxidation.

Carbon Deposition Characteristics and Regenerative Ability of Oxygen Carrier Particles for Chemical-Looping Combustion

For gaseous fuel combustion with inherent CO2 capture and low NOx emission, chemical-looping combustion (CLC) may yield great advantages for the savings of energy to CO2 separation and suppressing the effect on the environment. In a chemical-looping combustor, fuel is oxidized by metal oxide medium (oxygen carrier particle) in a reduction reactor. Reduced particles are transported to the oxidation reactor and oxidized by air and recycled to the reduction reactor. The fuel and the air are never mixed, and the gases from the reduction reactor, CO2 and H2O, leave the system as separate streams. The H2O can be easily separated by condensation and pure CO2 is obtained without any loss of energy for separation. In this study, NiO based particles are examined from the viewpoints of reaction kinetics, carbon deposition, and cyclic use (regenerative ability). The purpose of this study is to find appropriate reaction conditions to avoid carbon deposition and achieve high reaction rate (e.g., temperature and maximum carbon deposition-free conversion) and to certify regenerative ability of NiO/bentonite particles. In this study, 5.04% methane was used as fuel and air was used as oxidation gas. The carbon deposition characteristics, reduction kinetics and regenerative ability of oxygen carrier particles were examined by TGA (Thermal Gravimetrical Analyzer).

Development of sorbent manufacturing technology by Agitation Fluidized Bed Granulator (AFBG)

Thousands of ppmv of hydrogen sulfide included in coal gas should be reduced to less than a hundred ppmv in the case of IGCC to prevent a gas turbine from being corroded, and few ppmv to prevent the performance of electrodes from declining in the case of MCFC. In the present paper a laboratory scale AEBG (Agitation Fluidized Bed Granulator) is made and improved. The sorbent for the removal of hydrogen sulfide is produced using an agitation fluidized bed granulator (ZnO 1.5 mole+TiO2 l.0mole+bentonite 5.0 wt%). The techniques for fluidizing fine particles, classified in Geldart C group, in a fluidized bed are developed by installing an agitator blade in a fluidized bed granulator. The fine particles are fluidized and granulated successfully by using the techniques. Statistical, spectral and chaos analyses with granulated sorbent (100-300 Μm) are performed to investigate the hydrodynamics of granulates in a fluidized bed. The average absolute deviation, power spectral density functions, phase space trajectories, and Kolmogorov entropy obtained from pressure fluctuation are plotted as a function of fluidizing velocity. It is shown that the Kolmogorov entropy implying the rate of generation of information can be applied to the control of fluidization regimes.

Simultaneous experiments of sulfidation and regeneration in two pressurized fluidized-bed reactors for hot gas desulfurization of IGCC

Hot Gas Desulfurizarion for IGCC is a new method to efficiently remove H2S in fuel gas with regenerable sorbents at high temperature and high-pressure conditions. The Korea Institute of Energy Research did operation of sulfidation in a desulfurizer and regeneration in a regenerator simultaneously at high pressure and high temperature conditions. The H2S concentration at exit was maintained continuously below 50ppmv at 11,000 ppmv of inlet H2S concentration. The sorbent had little effect on the reducing power in the inlet gas in the range from 11% to 33% of H2. As inlet H2S concentration was increased, H2S concentration in the product gas was also increased linearly. The sorbent was maintained at low sulfur level by the continuous regeneration and the continuous solid circulation at the rate of 1.58× 10−3 kg/s with little mean particle size change.

Coal combustion characteristics in a pressurized fluidized bed

The characteristics of emission and heat transfer coefficient in a pressurized fluidized bed combustor are investigated. The pressure of the combustor is fixed at 6 atm. and the combustion temperatures are set to 850, 900, and 950 °C. The gas velocities are 0.9, 1.1, and 1.3 m/s and the excess air ratios are 5, 10, and 20%. The desulfurization experiment is performed with limestone and dolomite and Ca/S mole ratios are 1,2, and 4. The coal used in the experiment is Cumnock coal from Australia. All experiments are executed at 2 m bed height. In this study, the combustion efficiency is higher than 99.8% through the experiments. The heat transfer coefficient affected by gas velocity, bed temperature and coal feed rate is between 550-800 W/m2 °C, which is higher than those of AFBC and CFBC. CO concentration with increasing freeboard temperature decreases from 100 ppm to 20 ppm. NOx concentration in flue gas is in the range of 5-130 ppm and increases with increasing excess air ratio. N2O concentration in flue gas decreases from 90 to 10 ppm when the bed temperature increases from 850 to 950 °C.

Effect of Bed Insert Geometry on CO Conversion of WGS Catalyst in a Fluidized Bed Reactor for SEWGS Process

To enhance the performance of SEWGS system by holding the WGS catalyst in a SEWGS reactor using bed inserts, effect of bed insert geometry on CO conversion of WGS catalyst was measured and investigated. Small scale fluidized bed reactor was used as experimental apparatus and tablet shaped WGS catalyst and sand particle were used as bed materials. The cylinder type and the spring type bed inserts were used to hold the WGS catalysts. The CO conversion of WGS catalyst with the change of steam/CO ratio was determined based on the exit gas analysis. Moreover, gas flow direction was confirmed by bed pressure drop measurement for each case. The measured CO conversion using the bed inserts showed high value comparable to previous results even though at low catalyst content. Most of input gas flowed through the bed center side when we charged tablet type catalyst into the cylinder type bed insert and this can cause low CO2 capture efficiency because the possibility of contact between input gas and CO2 absorbent is low in this case. However, the spring type bed insert showed good reactivity and good distribution of gas, and therefore, the spring type bed insert was selected as the best bed insert for SEWGS process.

Attrition Characteristics of WGS Catalysts for SEWGS System

>> Attrition characteristics of WGS catalysts for pre-combustion CO2 capture were investigated to check attrition loss of those catalysts, to check change of particle size distribution during attrition tests, and to determine solid circulation direction of WGS catalysts in a SEWGS system. The cumulative attrition losses of two catalysts increased with increasing time. However, attrition loss under humidified condition was lower than that under non-humidified condition case for long-term attrition tests. Between two catalysts, attrition loss of PC-29 catalyst was higher than that of MDC-7 catalyst for long-term attrition tests. However, the MDC-7 catalyst generated much more fines than PC-29 catalyst during attrition. Therefore, we conclude that the PC-29 catalyst is more suitable for fluidized bed operation if we take into account the separation efficiency of cyclone. Based on the results from the tests for the effect of humidity on the attrition loss, we selected solid circulation direction from SEWGS reactor to regeneration reactor because the SEWGS reactor contains more water vapor than regeneration reactor.

A Study on Combustion & Flue Gas Characteristics of Coal at Pressurized Fluidized Bed Combustor

The characteristics of combustion and of emissions in pressurized fluidized bed combustor are investigated. The pressure of the combustor is fixed at 6 atm, and the combustion temperatures are set to 850, 900, and . The gas velocities are 0.9, 1.1, and 1.3 m/s. The excess air ratio is varied from 5 to 35%. The coal used in the experiment is Shenhwa coal in China. All experiments are executed at 2m bed height. Consequently, NOx & concentration in the flue gas is increased with incresing excess air ratio but concentration is decreased with incresing excess air ratio. CO concentration is maintained below 100ppm at over 15% of excess air ratio