Limestone Calcination Kinetics in Microfluidized Bed Thermogravimetric Analysis (MFB-TGA) for Calcium Looping

Abstract

Limestone calcination is an important part of calcium looping (CaL) technology and is critical to the design and operation optimization of fluidized bed reactors. However, obtaining a method of measuring the fast calcination kinetics in a fluidizing environment with isothermal conditions is still a challenge in the field of calcium looping. We address this challenge in this work and develop a new method of obtaining limestone calcination kinetics by injecting limestone particles into the hot fluidizing sands in a microfluidized bed thermogravimetric analysis (MFB-TGA) with a mass measurement resolution of 1 mg. The calcination characteristics of limestone are investigated at different particle sizes (150–1250 μm), temperatures (750–920 °C), and CO2 concentrations (0–30 vol.%). The experimental data measured from MFB-TGA were analyzed using a detailed model including surface reaction and intraparticle and external diffusion. The results show that the kinetics of limestone calcination measured by MFB-TGA are faster than those measured via regular TGA. This particle-injecting method of MFB-TGA provides a new experimental idea for measuring fast calcination kinetics occurring inside fluidized bed reactors and provides guidance on the application of CaL technology.

First Principle-based Rate Equation Theory for the Calcination Kinetics of CaCO3 in Calcium Looping

“…To confirm the result of the DFT calculation obtained in this study, k was obtained from eq 3; the natural logarithm of k versus 1000/T is depicted in Figure 6. The activation energy of calcination is revealed by the slope of the curve, equal to 184.68 kJ mol −1 , which sufficiently agrees with the results obtained in other studies, such as 186, 52 182, 17 and 181 ± 9 kJ mol −1 . 16 The equilibrium constant derived from the kinetics also demonstrated a good consistency with the thermodynamic results.…”

“…In an actual calcium-looping calcination reactor, the decomposition reaction is coupled with CO 2 diffusion and structural transformations, as shown in Figure . The decomposition of CaCO 3 commonly follows a shrinking core mode, as shown in Figure . − CaCO 3 decomposition occurs on the empty surface of the unreacted CaCO 3 core, and the solid products are metastable CaO*; a part of the CaCO 3 surface will be covered by metastable CaO*, and the CO 2 released from CaCO 3 decomposition must diffuse through the metastable CaO* layer to the gas phase. The metastable CaO* undergoes a nucleation step, that is, the cluster of metastable CaO* transforms into a CaO crystal.…”

“…Borgwardt also investigated the sintering characteristics of fresh porous CaO formed by CaCO 3 calcination in an atmosphere with CO 2 and H 2 O. , Silcox et al proposed a shrinking core model to describe the decomposition of CaCO 3 at the reactant–product interface, CO 2 diffusion through the growing CaO layer, and the CaO layer sintering. The shrinking core model is the most widely adopted model for studying calcination kinetics. − The influence of the particle size on the calcination kinetics can be sufficiently described by the shrinking core model. − In previous studies, the calcination rate is found to be inversely proportional to the concentration of CO 2 , that is, it exhibits linearity at low temperatures while at high temperatures it becomes nonlinear . Also, it is insignificant at low temperatures and exhibits a parabolic correlation at high temperatures .…”

Mechanism and kinetics of enzymatic degradation of polyester microparticles using a shrinking particle–shrinking core model

“…Integration of eqn (23) across the ash layer shows that at any given time, dN E is constant and proportional to the diffusion coefficient D.…”

“…Shrinking Particle (SPM) and Shrinking Core (SCM) models are widely used to describe transport phenomena in solid-fluid chemical reactions, [19][20][21][22][23][24][25] including polymer degradation. [26][27][28][29][30][31] These models by themselves can describe homogenous acid or base hydrolysis of polyethylene terephthalates, but cannot accurately describe the multi-step heterogenous enzymatic degradation of polymers.…”