Numerical research on thermal decomposition reaction kinetics of large particle size limestone

Abstract

Large particle size limestone (CaCO3) is an important raw material used in many branches of industry. For lacking of reaction parameters of large particle size limestone, numerical analysis method is used to study the decomposition reaction kinetics of limestone to understand the thermal decomposition reaction and obtain dominant parameters of large particle size limestone (20, 30, 40 mm). The results show that the decomposition reactions of three different particle sizes at 900~1000°C temperature conform to the phase interface reaction mechanism model (cylindrical symmetry), and the reaction activation energy E is ranging from 30 to 34 kcal/mol and the pre-exponential factor lnA is ranging from 7.5 to 9s, which are conference data for practical engineering. Introduction Limestone (CaCO3) is an important raw material used in many branches of industry, such as, construction, metallurgy, pharmaceuticals and paper industry. Thermal decomposition reaction of limestone plays an important role in practical production. Usually, the main methods of limestone decomposition reaction mechanism are thermo-gravimetric and numerical analysis methods. Thermo-gravimetric method is only applicable to researching limestone with particle size less than 10 mm, restricted by experimental equipments. At present, the design of lime kiln and the control of lime production process mostly depend on numerical method. There are some researchers, who have studied on limestone decomposition reaction of small particle size limestone. The limestone of 45-90 microns particle size at 750-900°C temperature was performed by S.J.Wang and J.D.Lu investigating the limestone decomposition reaction. Chen Hai analyzed the limestone of 1-5mm particle size, and the obtained limestone decomposition reaction mechanism. However, there is very less published information available for limestone decomposition reaction of large particle size. Limestone with large particle sizes (more than 10 mm) are widely used in industrial production. Some key parameters are involved in numerical analysis of large particle sizes, which provide dominant parameters for setting up the thermal simulation equation and realizing online simulation in lime kilns. In this work, the mechanism of thermal decomposition reaction kinetic parameters of 20, 30, 40 mm diameter limestone are studied at temperature 900, 950, 1000°C using numerical analysis method. Based on the results of this work, further establishing thermal equation and selecting reaction conditions can be obtained. Numerical analysis method For exploring limestone decomposition reaction kinetics, the numerical equation is firstly solved, concentration profiles under different temperature and particle size are obtained. Secondly, the decomposition reaction mechanisms are determined and related kinetic parameters are dominated. Concentration profile between limestone decomposition rate and time The first step of decomposition reaction research is obtaining concentration profiles between decomposition rate and time, which are acquired through numerical analysis equation. The thermal 5th International Conference on Advanced Materials and Computer Science (ICAMCS 2016) © 2016. The authors Published by Atlantis Press 1032 decomposition reaction consists of five sub-processes including: (a) heat transfer from the ambient to the solid surface, (b) heat conduction from the surface to the reaction front, (c) chemical reaction at the front, (d) diffusion of CO2 through the porous oxide layer to the surface, (e) the mass transfer of CO2 into surroundings. Based on limestone decomposition reaction five sub processes, Szekely et al. [4] and Kainer et al. [5] derived a coupled differential equations for the calculation of conversion degree X and the decomposition temperature Tf as followed: dX/dt=Kc·ΔhR/(3·(Tg-TF)·(rs/α+rs/λ·((1-X)-1)) ) (1) dX/dt=Kc·Rc·TF/(3(Peq-Pg)·(rs/β+rs/DP·((1-X))+ rs/k·(1-X))) (2) where t is time, X is decomposition rate of limestone, ds is the diameter of the particles, α,λ,κ,Ds,β are performance parameters of limestone decomposition process. With Eqs.(1)-(2), the conversion degree of limestone can be calculated as function of time, which provides foundation data for thermal decomposition reaction kinetics mechanism. Reaction kinetics mechanism model Kinetics research is to obtain the three factors of decomposition reaction kinetics: reaction mechanism model function and former factor and activation energy. At present, there are many kinds of limestone decomposition reaction kinetics model, which can be generally divided into three categories: random nucleation and subsequent growth, diffusion control, phase interface reaction mechanism model. Determining the limestone reaction mechanism model is the dominant part of decomposition reaction kinetics. Among the research of reaction mechanism, reaction mechanism model and the corresponding kinetic parameters are obtained by linear fitting method which combined with the conversion degree. Reaction kinetics equations of solid particles are as follows: