Thermal Conversion of Pine Wood and Kinetic Analysis under Oxidative and Non-Oxidative Environments at Low Heating Rate

Lelis Gonzaga Fraga,Senhorinha Teixeira,José Teixeira,João Silva,Delfim Soares,Manuel Ferreira

doi:10.3390/wef-06921

Abstract

Atmosphere is one of the most significant factors in the thermal decomposition of biomass. In domestic or industrial biomass boilers, ambient oxygen concentration varies through time, which means that the reaction will change from pyrolysis to combustion. In this way, to analyze and compare each thermochemical conversion process, a simple analytical method, the non-isothermal thermogravimetric analysis, is carried out under oxidative (air) and non-oxidative (argon) environments at 10 °C/min and as a function of different flow rates (2 to 150 mL/min). Additionally, this work was complemented by a kinetic analysis considering a first-order reaction to each conversion stage and using the Coats–Redfern method. The effect of the atmosphere on the thermal decomposition behavior was evident. It was observed that the thermal decomposition of pine wood particles varied from three to two stages when the oxidative or inert atmosphere was applied. The presence of oxygen changes the mass loss curve mainly at high temperature, around 350 °C, where char reacts with oxygen. The maximum mass loss rate from experiments with the oxidative atmosphere is 15% higher than in an inert atmosphere, the average char combustion rate is approximately 5 times higher and the heat released reaches levels 3.44 times higher than in an inert atmosphere. Ignition and combustion indexes were also defined, and results revealed that particles are ignited faster under oxidative atmosphere and that, on average, the combustion index is 1.7 times higher, which reinforces the more vigorous way that the samples are burned and how char is burned out faster in the experiments with air. Regarding the kinetics analysis, higher activation energies, and consequently, lower reactivity was obtained under the oxidative atmosphere for the second stage (~125 kJ/mol) and under the inert atmosphere for the third thermal conversion stage (~190 kJ/mol).

Highlights

Due to increasing concerns about the limited availability of fossil fuels and pollutant emissions by conventional systems, sustainable heat and power generation from biomass are of great interest for scientific and industrial communities given its potential as an alternative and renewable energyProceedings 2020, 58, 23; doi:10.3390/WEF-06921 www.mdpi.com/journal/proceedingsProceedings 2020, 58, 23 source [1]
This is one of the most significant factors in the thermal decomposition of pine wood particles since in industrial furnaces, ambient oxygen concentration varies through time, which means that the reaction will change from pyrolysis to combustion
It was observed that the thermal decomposition of pine wood particles varied from three to two stages in the presence of an oxidative or inert atmosphere respectively; The presence of oxygen changes the mass loss curve mainly at high temperature, around 350 °C, where char reacts with oxygen and promotes the degradation of the remaining mass

Summary

Introduction

Due to increasing concerns about the limited availability of fossil fuels and pollutant emissions by conventional systems, sustainable heat and power generation from biomass are of great interest for scientific and industrial communities given its potential as an alternative and renewable energyProceedings 2020, 58, 23; doi:10.3390/WEF-06921 www.mdpi.com/journal/proceedingsProceedings 2020, 58, 23 source [1]. It is important to understand the physical and chemical processes involved in biomass combustion, which is the main conversion process used in industrial boilers to produce energy [2] In this thermochemical conversion process, the main step is devolatilization and its knowledge as the knowledge of its kinetics can help provide a better understanding of the design and optimization of biomass combustion systems [3]. To understand this phenomenon, the thermogravimetric analysis (TGA) technique is widely applied to study the thermal degradation of biomass samples and in kinetic studies, due to the ease in sample weight loss and temperature profile along the time [4,5]. The characteristic temperatures (i.e., onset, peak and offset temperatures of the maximum weight loss rate) are important parameters to establish the residence time desired for the biomass in the combustion chamber and can be obtained by the differential thermogravimetric data

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