Theoretical exploration of char formation during lignin pyrolysis

Abstract

In this thesis, we explore the formation of char during lignin pyrolysis. This is accomplished using computational chemistry to study the energetics and mechanisms of reactions involved in lignin char formation.

In Chapter 1, the background for the project is given, highlighting the global climate and energy crises, as well as providing an overview of biomass, biofuels, and biomass thermochemical conversion techniques. Lastly, the research gap is outlined and the key research topic of the thesis is introduced.

In Chapter 2, we assess the stability of a range of lignin derived phenolic bio-oil compounds through the calculation of bond dissociation enthalpies. Here we show that methoxy functional groups exhibit much lower bond dissociation enthalpies than other groups found in lignin derived phenolic compounds which is in agreement with literature reports of their scarcity at higher pyrolysis temperatures. Conversely, unsaturated functionalities exhibit high resistance to homolytic cleavage and are more thermally stable.

In Chapter 3, we propose a reaction mechanism for the formation of aromatic char clusters from the products of lignin decomposition. Here, through investigation of sixteen parallel pathways, we show clearly how unsaturated lignin derived monomers may concatenate through cycloaddition reactions and subsequently aromatise through elimination reactions to yield polycyclic char clusters.

Chapter 4 is concerned with the aggregation of these char clusters through a continuation of the reactions reported in the previous chapter. By employing the Same Level Different Basis approach, the growth of condensed char clusters up to eleven rings is modelled and the impact of cluster size on reaction barriers is assessed.

Chapter 5 contains concluding remarks that draw upon the results reported in Chapters 2-4. This chapter also proposes new avenues for the investigation of lignin char formation and the potential impact of the work contained herein to other closely related fields.

Date of Award	Dec 2020
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Northern Ireland Department for the Economy
Supervisor	Beatrice Smyth (Supervisor), Brian Falzon (Supervisor) & Xiaolei Zhang (Supervisor)