Detailed kinetics of pyrolysis, gasification and combustion of bio-fuels

The environmental apprehension towards the combustion of fossil fuels together with the growing concern on waste materials drive the interest in the exploitation of biomasses. Among possible solid fuels, biomasses are a very important energy resource and they are gaining an increasingly important role worldwide as a widely diffused, available, and renewable energy source. Biorefinery process technologies include thermochemical (gasification, pyrolysis), biochemical (fermentation), or chemical (chemical synthesis) pathways. There is considerable interest in promoting the use of butanol (C4H9OH) as an alternative to ethanol. Butanol can be derived from lignocellulosic materials and it has some advantages, as transportation fuel component, when compared with ethanol: it is less corrosive, has a lower vapor pressure, higher energy density, and its octane rating is similar to that of gasoline. These limitations are expected to improve in the near future. Commercial biodiesel fuels are produced via transesterification of triglycerides extracted from a variety of biolipid feedstocks. These biofuels are mostly composed of unsaturated methyl esters, methyl oleate (C19H36O2) and methyl linoleate (C19H34O2), with minor quantities of saturated components such as methyl palmitate (C17H34O2) and methyl stearate (C19H38O2). Relevant amounts of methyl linolenate (C19H32O2), characterized by three double bonds, are also present. The aim of this dissertation is the mathematical description of the combustion and gasification of bio-fuels, both in gas and solid phase. Thus, this work has two different sections. The first one deals with the development of the kinetic models of pyrolysis and oxidation of the four isomers of butanols and methyl esters (i.e. bio diesel) up to methyl decanoate. Reactivity and products distribution during combustion in different conditions have been validated on a wide range of different conditions. Laminar flame modelling is properly emphasised at the end of this first section. In the second section, we developed a reactor and particle model able to describe a solid fuel gasifier. This problem is multi-scale, multi-component and multi-phase. The coupling of reactor and particle models with a detailed kinetics (both in gas and solid phase) represents the novelty of this approach. A validation in fast pyrolysis conditions is shown and parametric analyses on the production of bio-oil are presented. A validation in a fixed bed experimental reactor is introduced and finally an application of the whole gasifier is described. Despite the computational difficulties, the approach developed in this work allows an overall and comprehensive understanding of the problem. Indeed, only the coupling of detailed kinetic models, like the ones developed in the first part of this thesis, with the models at particle and reactor scales allows to distinguish and to analyze the role of chemical kinetics and transport phenomena and to properly design industrial units.

In questo lavoro vengono trattati gli aspetti cinetici della modellazione matematica di pirolisi, gasificazione e combustione di bio-combustibili. La tesi è strutturata in due sezioni, la prima riguardante lo sviluppo di modelli cinetici dettagliati dell’ossidazione degli isomeri del butanolo e di modelli semi-dettagliati per descrivere la combustione dei metil esteri. La convalida dei modelli sviluppati è stata effettuata su un ampio spettro di dati sperimentali reperiti dalla letteratura scientifica. Inoltre, ampio spazio è stato riservato in questa prima sezione all’analisi del comportamento del modello nella determinazione delle velocità di fiamma. Analisi di natura cinetica sono presentate in dettaglio. La seconda parte riguarda la modellazione matematica di un gasificatore a letto fisso di biomassa. Le cinetiche di devolatilizzazione del solido, così come le reazioni secondarie in fase gassosa, vengono accoppiate con i modelli fisici alla scala della particella e del reattore. Il problema complessivo risulta essere multi-scala, multi-componente e multi-fase. Primi risultati di convalida sono stati ottenuti in condizioni di pirolisi veloce per la produzione di bio-olio. Infine, risultati preliminari della modellazione di un gasificatore di biomassa sono presentati. L’oneroso tempo di calcolo richiesto rappresenta il principale limite di questo approccio. Tuttavia, l’analisi dettagliata proposta, sia in termini cinetici chimici che nella modellazione del sistema alla scala della particella e del reattore, consente di poter discernere gli aspetti prettamente chimici da quelli legati ai fenomeni di trasporto.