Kinetics Investigation on the Pyrolysis of Lignite

The pyrolysis has been well known as a clean technology of making full use of the lignite resource. Commercializing the relevant process partially based on the cheap Computation Fluid Dynamics (CFD) simulation became possible if the relatively accurate and general reaction rate expression could be defined. However, the distributed activation energy model (DAEM), which was widely used to predict the coal pyrolysis, needed multiple integration to evaluate the reaction rate, making it difficult to be coupled into the species transport equations in CFD. In order to avoid the numerical integration, the lignite pyrolysis was assumed to be comprised of multiple parallel first order reactions in this work, and the total reaction rate was written as the sum of general Arrhenius equations. The kinetics parameters for each assumed reaction, namely, pre-exponential factor, activation energy and mass fraction, were obtained by fitting the thermogravimetry (TG) experimental data. The results showed that the general Arrhenius equations can more accurately predict the pyrolysis of lignite than DAEM. And then, based on the understanding that the chemical bonds broken in the pyrolysis would be same while the relevant fraction would be varied with the different kinds of lignite, we tried to use the activation energy and pre-exponential factor from one kind of lignite to fit the other three kinds of lignite, and got an acceptable result, which could validate the above theoretical analysis to some extent. In addition, for investigating the effect of particles diameter and heat transfer coefficient on the pyrolysis behavior, the model to predict the pyrolysis in a coal particle was developed based on the above kinetics expression. The results showed that the fluidized bed was a better choice to commercialize the lignite pyrolysis and the particles size should be smaller than 3 mm. Also, compared with DAEM, the computational time can be reduced from 51.9 s to 3.8 s under the same conditions, making it possible to design and optimize the relevant pyrolysis reactor based on the cheap CFD analysis. This investigation provides the reference of simulating the pyrolysis by coupling the Arrhenius kinetics reaction expressions into the species transport equations in CFD.

Keywords: Lignite pyrolysis, Kinetics, CFD, DAEM, Fluidized bed