Title

Proposal of a representative chemical composition for fast pyrolysis lignocellulosic bio-oil and estimation of the thermodynamic properties of the bio- oil compounds

Conference Dates

June 16-21, 2019

Abstract

One of the most promising biorefinery platforms is based on the production of bio-oil via fast pyrolysis of lignocellulosic biomass. Lignocellulosic pyrolysis bio-oil is a complex mixture of water and organic compounds coming from the devolatilization and the thermal ejection of the main biomass components (cellulose, hemicellulose, lignin and extractives), but also from secondary reactions of these primary pyrolysis products. The very diverse chemical reactions and phenomena that take place during the pyrolysis process, together with the different structure of biomass constituents, make the pyrolysis bio-oil a mixture of very different compounds, going from light volatile compounds to oligomers and presenting also a wide variety of functional groups. Ultra high resolution mass spectrometry techniques enable the discrimination of thousands of peaks, assigned each one to a unique CcHhNnOoSs elemental formula[1]. In the pyrolysis bio-oil biorefinery platform, separation and refining processes are of utmost importance. For this reason, there is an extensive research work on refining treatments of bio-oil[2] and an incipient research work on separation processes[3]. The complexity of the own processes and the multitude of parameters affecting them make their experimental study costly and tedious. The simulation of shortcut refining and separation methods, but also of the own pyrolysis process, could allow assessing the suitability of a certain method or the accomplishment of parametric studies. The availability of a representative chemical composition of the pyrolysis bio-oil and also of the thermodynamic properties of the proposed compounds is a necessary step for the success of any of these simulations. In this context, the two objectives of this work were 1) to propose a simple but representative chemical composition of lignocellulosic pyrolysis bio-oils and 2) to estimate some of the thermodynamic properties of the compounds included in the proposed composition.

Literature[4] and own experimental data have been used to propose the chemical composition. In this way, more than 30 different compounds have been included in the proposed composition, such as, main volatile compounds (e.g. acetic acid, glycoaldehyde, acetol or levoglucosan), some HPLC detectable sugars (galactose or cellobiosan) and three different types of oligomers (pyrolytic lignin, pyrolytic humin and hybrid oligomers). The enthalpy of formation of the non-tabulated compounds included in the composition have been estimated using mainly a group contribution-based method[5]. For the calculation of the enthalpy of formation, it has also been necessary the estimation of other thermodynamic properties, such as the boiling point, the critical pressure and temperature, the enthalpy of vaporization or the Gibbs free energy, among others. Examples of the estimated enthalpies of formation values are: glycoaldehyde = -379.9 kJ·mol-1, levoglucosan = -981.2 kJ·mol-1, hybrid oligomers = -7774.2 kJ·mol-1.

The suitability of the proposed pyrolysis bio-oil composition has been checked using two different ways: 1) the total concentration of some functional groups (carbonyl, carboxyl, hydroxyl, phenolic) determined experimentally by other authors[4] has been compared to their theoretical concentration calculated using the structural formulas of the compounds proposed, and 2) the liquid enthalpy of formation of the pyrolysis bio-oil calculated from the experimental values of heating value and ultimate analysis has been compared to the liquid enthalpy of formation calculated as the weighted average of the estimated enthalpies of formation of the proposed compounds. The differences between the experimental and the estimated values of functional groups concentration and of bio-oil enthalpy of formation were lower than 10%.

[1] J. Hertzog, V. Carré, Y. Le Brech, A. Dufour, F. Aubriet, Energy & Fuels 2016, 30, 5729-5739.

[2] aA. H. Zacher, M. V. Olarte, D. M. Santosa, D. C. Elliott, S. B. Jones, Green Chemistry 2014, 16, 491-515; bG. W. Huber, S. Iborra, A. Corma, Chemical Reviews 2006, 106, 4044-4098.

[3] A. A. Kiss, J.-P. Lange, B. Schuur, D. W. F. Brilman, A. G. J. van der Ham, S. R. A. Kersten, Biomass and Bioenergy 2016, 95, 296-309.

[4] F. Stankovikj, A. G. McDonald, G. L. Helms, M. Garcia-Perez, Energy & Fuels 2016, 30, 6505-6524.

[5] K. G. Joback, M. S. Thesis, Massachusetts Institute of Technology (Cambridge, MA), 1984.

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