(446b) Global Kinetics of Species Formation during High Temperature Pyrolysis of Coal and Biomass in CO2 Environment

High temperature pyrolysis releases hydrocarbon gases. Due to fast secondary reactions taking place during high temperature pyrolysis, the concentration of these gases decreased as the pyrolysis temperature was increased from 1300 to 1500°C. An attempt to express the apparent rate of formation of these species from a global kinetics perspective, using experimental data would yield negative activation energies, based on Arrhenius law. This seems to be unrealistic in view of the fundamental definition of activation energy, which is the energy barrier to surmount for a reaction to take place. However, it is worth noting that the Arrhenius rate law is a fit for a single step reaction. The temperature variation of the apparent rate constant supposedly obeying a power-law (Arrhenius rate law in this case) and agreeing with experimental data helps determine what is called an “apparent activation energy” for individual species formation. The exact mechanism leading to the formation of each species is unknown. Although fulfilling the expectation for mathematical modeling of pyrolysis reaction kinetics, the negative activation energies obtained for these species implies that there are many important reactions taking place that need to be investigated further.

Only few hydrocarbons, the lightest ones, are expected in high temperature pyrolysis environment. It is well established that the higher the reactor temperature the less methane and other hydrocarbons will be present. Moreover the rate of conversion of these products by secondary reactions is faster. Hydrocarbons species present in the high temperature pyrolysis products is therefore a balance between their formation and decomposition. Methane is unstable in terms of its elements from 530 ℃, but remains as the most stable hydrocarbon up to 1030°C.

Pyrolysis study of two biomass fuels (Switchgrass and Pine Sawdust) and coal was conducted in a lab scale entrained flow reactor at the Pennsylvania State University while measuring the concentrations of gases in real time. The measured concentration of methane during pyrolysis of these fuels at 1300, 1400 and 1500℃ showed a steady decrease with increase in temperature.

The concentrations of ethylene and acetylene were also found to decrease with the increasing pyrolysis temperature. This observation does not conform to the species formation according to the primary pyrolysis mechanism, which should increase with temperature increase. Therefore the decrease of methane, ethylene and acetylene in the temperature range of 1300 to 1500℃ is certainly due to their conversions by secondary pyrolysis reactions. This successive conversion of hydrocarbons is in conformity with the mechanism of conversion of hydrocarbons to polycyclic aromatic hydrocarbon (PAH) and soot. 

Two conversion routes for methane have been postulated by these experiments

• The direct pyrolysis of methane and
• Reaction of methane with CO₂, which is abundant in the our experimental environment

Given the complexity of the reactions taking place in this environment, and the lack available literature data at these conditions, a phenomenological model has been developed to match the kinetic equations and the measured data on species. A mass-based derivation of the species kinetics is adopted as it makes the analysis more coherent. The data generated was used to simulate the reaction mechanism in Ansys Fluent, producing good agreement with experimental results.