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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - PYROCHEM (Biopolymers 13C tracking during fast pyrolysis of biomass-A 2-level mechanistic investigation)

Teaser

Summary

To reduce the large consumption of fossil resources that have contributed to climate change, Pyrochem has proposed to investigate the fundamentals of biomass fast pyrolysis, a technology that allows production of advanced biofuels. It has long been demonstrated that plant biomass is efficiently converted into liquids via fast pyrolysis. The mixture of aerosols/vapours is then rapidly quenched resulting in a relatively high yield of bio-oil. In addition to the yield, the distribution of products determines the quality of bio-oils, which critically depends on biomass type and temperature-time history. Despite the great added value of fast pyrolysis bio-oil, their application as transport fuels remains limited. Indeed, the pyrolytic liquid is a complex mixture of reactive organic compounds which properties are difficult to control. The relative proportions of cellulose, hemicelluloses and lignin in biomass feedstocks directly influence the chemical reactivity of biomass and have a significant impact on the composition of the bio-oil and its properties. While it is still difficult to predict how the lignocellulosic distribution impacts fast pyrolysis throughputs and the quality of its products, further insight in pyrolysis chemistry is required to (i) reveal key mechanistic details of the formation of bio-oil and (ii) precise the impact of the interactions between lignocellulosic biopolymers on bio-oil\'s quality.

Work performed

The PYROCHEM project was divided into 4 experimental phases: (i) deconstruction of raw and 13C-enriched biomass; (ii) fast pyrolysis (FP) of raw and extracted materials under controlled conditions; (iii) analysis of pyrolysis products using in-depth spectrometric techniques and (iv) theoretical evaluation of primary FP reactions.
-1st: The selection of the extraction techniques is critical as the purity level of extracted biopolymers relies on it. In addition to displaying high 13C levels, the biopolymers must have a sufficient purity and limited structural changes. The impact evaluation of extraction techniques on the chemical modifications of biopolymers has been limited to alkaline extraction and ultrafiltration techniques applied to hemicelluloses. The hemicellulosesâ€™ chemistry revealed by the analysis using the specificities of monoclonal antibodies have indicated inherent chemical changes. The characterization of â€˜primaryâ€™ volatiles obtained from the FP of isolated hemicelluloses are being combined with results from the theoretical study (4th).
-2nd: One of the challenges of this project was to reach steady-state conditions during the FP and studying kinetic-controlled chemical reactions. It appeared that both design specifications of microreactors did not permit to reach pure kinetic regime. As a direct consequence, the chemistry of volatiles is the result of the overlap of mainly primary reactions and few inevitable secondary reactions. Despite the technical limitations due to the design of microreactors, the collection of a reasonable amount of â€˜primaryâ€™ condensates obtained from the FP of biomass has been possible.
-3rd: The in-depth characterization of condensable pyrolysis products has been done by means of mass spectrometry (MS) and nuclear magnetic resonance (NMR) techniques combined with the use of 13C enriched materials. The use of the pyroprobe has allowed the on-line analysis by MS of hot volatiles released during the â€˜primaryâ€™ stage of FP. Comparison between the chemical composition of volatiles from the mixture of unlabelled and carbon-13 enriched materials indicated that no scrambling (no exchange between carbons 12 and 13) occurred during the primary stage of degradation; revealing the predominance of non-ionic chemical reactions with internal molecular rearrangements. In addition, the condensates were analysed using NMR methodology. The liquid 13C NMR spectroscopy, in association with the use of 13C-enriched material, permitted the technical limitations related to the low natural abundance of 13C to be overcome.
-4th: The theoretical modelling of pyrolysis mechanisms was undertaken with Gaussian package. Significant progress has been made in understanding how to appropriately describe the molecular system. The scope of the investigation has been reduced to the pyrolysis of hemicelluloses, the biopolymers the least studied. The geometry optimization of a representative fragment has been carried out by using the density functional theory methods. Pericyclic mechanisms have been investigated as primary depolymerisation reactions.
The kinetics aspect has been also apprehended following new and robust experimental guidelines using thermogravimetry and a MATLAB program. This study assesses the global reactivity of extracted biopolymers; providing clues on how a refined but still global description of biomass reactivity can be used to improve the understanding of pyrolysis events.

Final results

Science-based impact: This project has led to the publication of two peer-reviewed publications introduced during international meetings focussed on Pyrolysis fundamentals. There was a consensus on the importance of this topic as being one of the key drivers that will ease the integration of pyrolysis technologies to other existing processes and help with the selection of feedstocks pre-treatments and post-treatments of pyrolysis products.
A number of experts in pyrolysis have confirmed the relevance of the Pyrochem projectâ€™s objectives and the weight of its findings for the scientific community in particular on the difficult exercise, which is scaling up pyrolysis technologies while maintaining appropriate selectivity and efficiency figures. Finally, the applicability of the kinetic and characterization approaches to widespread pyrolysis conditions needs to be tested. Published kinetics guidelines have already been used by a number of researchers in our group and other Research groups worldwide; thus demonstrating the usefulness of this approach. The use of labelled compounds combined with 13C NMR spectroscopy should definitively become a useful tool to assess the stoichiometry of pyrolysis chemical reactions.

Socio-economic impact: In addition to revealing important scientific findings, the Pyrochem project has also played a major role in reinvigorating the interest and importance of fundamental Pyrolysis chemistry in Europe. By her impressive networking activities and the broad spectrum of dissemination activities over these two years, Dr. Marion Carrier has regularly communicated the findings related to the progress Pyrochemâ€™s project to a non-expert and expert audiences; thus maximizing the impact of the Pyrochem project. Varied dissemination and communication actions have been undertaken: the writing of popular articles with national and international coverage, the participation in a series of public engagement activities to raise the interest of the general public in biomass FP to promote the green and sustainable context of the Pyrochemâ€™s project, and the organization of seminars and talks at different partnersâ€™ institutions and international congresses. On the technological point of view, a unique method has been established and will offer a systematic and reliable molecular mapping of fast-pyrolysis bio-oil (FPBO); thus assessing the intricate composition of FPBO according to the operating conditions and type of reactor, but also ensuring the consistent characterization of certain commodity chemicals.