Double Distribution Activation Energy Model as Suitable Tool in Explaining Biomass and Coal Pyrolysis Behavior

Abstract

Understanding and modeling of coal and biomass pyrolysis assume particular importance being the first step occurring in both gasification and combustion processes. The complex chemical reaction network occurring in this step leads to a necessary effort in developing a suitable model framework capable of grasping the physics of the phenomenon and allowing a deeper comprehension of the sequence of events. The aim of this work is to show how the intrinsic flexibility of a model based on a double distribution of the activation energy is able to properly describe the two separate steps of primary and secondary pyrolysis, which characterize the thermochemical processing of most of the energetic materials. The model performance was tested by fitting the kinetic parameters from experimental data obtained by thermogravimetric analysis of two materials, which represent very different classes of energy source: a microalgae biomass and a sub-bituminous coal. The model reproduces with high accuracy the pyrolysis behavior of both the materials and adds important information about the relative occurring of the two pyrolysis steps.

Triple‐Gaussian distributed activation energy model for the thermal degradation of coal and coal chars under a CO2 atmosphere

“…The three steps are (a) primary pyrolysis (tar and light hydrocarbon gas formation), (b) secondary pyrolysis (char condensation, cross‐linking reactions, and further gas production), and (c) gasification (newly generated char‐CO 2 reaction). The former two steps are the same for 2‐DAEM, and the third one is reported in the literature . In the 3‐DAEM, three sets of parallel reactions occur, sharing the same pre‐exponential factor but not the same distributed activation energy.…”

“…This approach can also be seen in the thermal decomposition of biomass under a CO 2 atmosphere by using multi‐Gaussian DAEM . A value of k 0 = 5 × 10 11 s −1 has been found to be adequate . On this basis, it has been assumed in this study that thermal decomposition of all samples share the same pre‐exponential factor k 0 with a fixed value of 5 × 10 11 s −1 .…”

“…A problem involving the estimation of kinetic parameters is the compensation effect of pre‐exponential factors k and E , which provide an equally good fit to experimental data. It is for this reason that the value of k is often fixed and selected in a manner consistent with the transition‐state theory ( k 0 ≈ 10 11 −10 16 s −1 ) . This approach can also be seen in the thermal decomposition of biomass under a CO 2 atmosphere by using multi‐Gaussian DAEM .…”

“…The distributed activation energy model (DAEM) has been considered as the most comprehensive model for analyzing pyrolysis . Much research has focused on coal pyrolysis under an inert atmosphere by TGA . Pyrolysis under a N 2 atmosphere can be divided into two steps: The first occurs at low temperature, which releases light gas and tar and is referred to as primary pyrolysis.…”

“…Much research has focused on coal pyrolysis under an inert atmosphere by TGA . Pyrolysis under a N 2 atmosphere can be divided into two steps: The first occurs at low temperature, which releases light gas and tar and is referred to as primary pyrolysis. The second step occurs at high temperature, which includes repolymerization of coal molecules to produce char and is referred to as secondary pyrolysis.…”