CO₂ Gasification of a Lignite Char in Microfluidized Bed Thermogravimetric Analysis for Chemical Looping Combustion and Chemical Looping with Oxygen Uncoupling

Abstract

Chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU) are novel combustion technologies with low energy penalty CO2 capture. In a CLC/CLOU unit, the gasification of char is the rate-controlling step as a result of the slow gasification kinetics of char under a CO2 and H2O atmosphere. Reliable measurements of char gasification kinetics are absolutely necessary for CLC/CLOU reactor design, operation, and optimization and are the focus of this paper. To solve the problems of measuring gasification kinetics using thermogravimetric analysis (TGA) and fluidized bed (FB) reactors with gas measuring, a microfluidized bed thermogravimetric analysis (MFB–TGA) method was developed and reported in this paper to investigate the CO2 gasification of a lignite char. The feasibility of the novel MFB–TGA approach was tested, and its stability and accuracy were validated. MFB–TGA was then used to study the CO2 gasification of a lignite char. First, the CO2 gasification kinetics of the lignite char in MFB–TGA were compared to those obtained using a TGA-Q500 instrument. The char gasification kinetics in MFB–TGA were found to be much faster than those in the TGA-Q500. Second, the effect of the particle size on the gasification kinetics was investigated, and it was found that 900–1250 μm was the threshold size range, below which there was no obvious particle size effect on the kinetics. However, for particle sizes larger than 900–1250 μm, the effect of the particle size on the kinetics was significant. Then, using 900–1250 μm particles, the influence of the temperature, CO2 concentration, and O2 concentration was studied, and the experimental results obtained from MFB–TGA showed that the lignite char could be fully converted within 300 s at 900 °C with 75 vol % CO2. The time for the full conversion of the char could be reduced to 190 s when the gasification temperature was increased to 950 °C or when 3 vol % O2 was present in the atmosphere. Finally, a predictive kinetic model was developed from the experimental results, which could predict both the conversion of carbon versus time and the reaction rate versus carbon conversion well.

CaO carbonation kinetics determined using micro-fluidized bed thermogravimetric analysis

“…The response time, τ, was determined by both the response of the weight transducer and the response of the reactor. In this work, the weight transducer recorded data every 0.2 s. Additionally, in our previous work [46], the response time of the weight transducer was confirmed to be much less than 1 s for a normal reaction. Therefore, the response of the reactor may determine the total system response time.…”

“…The micro-fluidized bed thermogravimetric analysis apparatus (shown in Fig. 1(a)) that was used in this work has been previously reported by Li et al [46]. The apparatus consisted of a quartz reactor, an electric furnace, a gas supply system, and a measuring system.…”

“…When the gas velocity is too low, the fluidization condition is not good enough. When the gas velocity is too high, the mass measurement would be quite unstable, as illustrated detailedly in our previous work [46]. Therefore for all experiments, the volumetric flow rates of the reaction gas and inert gas were set to achieve a superficial velocity of 3.5-4 U mf [47].…”

“…The effect of the operational gas velocity on MFB-TGA is a very important factor that should be carefully selected. Based on our previous work [46], the apparent mass change due to the gas-solid reaction could be measured accurately if the gas velocity was controlled below a proper value, namely 6 U mf . Therefore, in this study, the gas velocity (3.5-4 U mf ) was selected for the mass measurement.…”

A First-Principles Microkinetic Rate Equation Theory for Heterogeneous Reactions: Application to Reduction of Fe2O3 in Chemical Looping

“…In the top-down method, the redox kinetics of a selected oxygen carrier is first measured by some setups such as a thermogravimetric analyzer (TGA), − microfluidized bed thermogravimetric analyzer (MFB-TGA), − and fluidized bed reactor with measuring gas signals. − The measured experimental data are then analyzed by using a simple gas–solid reaction model, and the kinetic parameters are obtained by fitting the experimental data. − With this model and obtained kinetic parameters, the circulation rate and bed inventories of the Fe-based oxygen carrier are calculated for the design and operation of a CLC process. This kind of top-down method does not consider any surface elemental mechanism occurring at the solid surface and cannot provide a microscopic insight at the atom scale. − …”