Production de biocombustibles par densification et pyrolyse de déchets végétaux : vers une approche innovante de bioéconomie circulaire

Abstract

To address energy transition challenges and the need to reduce greenhouse gas (GHG) emissions, the beneficiation of lignocellulosic biomass provides a promising route for producing renewable biofuels. This thesis examines densification and pyrolysis processes to establish the links between biomass properties, operating parameters, and the performance of the resulting biofuels. First, an industrial-scale study was conducted to convert mixtures of rice husk and wood (neem, ayous, mahogany) into densified fuels using a mechanical piston press. Evaluating the feeding speed, input particle size, and blending ratio showed that these parameters significantly influence briquette density, water resistance index, heating value, and combustion rate. Wood-rich formulations outperformed conventional firewood. A fundamental laboratory study then evaluated the impacts of compaction force, particle size, and binary/ternary blend compositions on the properties of single-layer and multi-layer fuels produced from spruce sawdust, miscanthus, and wheat straw. Decreasing the particle size while increasing both the compaction force and the wood content improved briquette quality by lowering ash content and increasing volatile matter and heating values. Furthermore, an innovative multi-layer briquette concept was proposed to produce fuels with superior performance (density, shatter resistance, ignition time, and combustion duration) compared to conventional single-layer ones. Regarding pyrolysis, wood-rice husk blends were subjected to non-isothermal thermogravimetric analysis to study their kinetics via two isoconversional approaches. Wood, characterized by lower apparent activation energies, proved to be more reactive. Synergistic and inhibitory effects still highlighted complex interactions between biopolymers and minerals during the treatment of these wood/rice husk blends, with silica delaying volatile release and potassium catalyzing decomposition reactions. These biomasses were subsequently pyrolyzed in a pilot-scale auger reactor to evaluate the influence of the temperature and blend composition. Maximum bio-oil yields occurred at 500 °C, while increasing the temperature and the rice husk content promoted syngas and biochar production, respectively. Characterizing the oily fractions using ultra-high resolution mass spectrometry revealed thousands of distinct molecular formulas, dominated by the CHO and CHON families. A stable molecular core and high water contents resulted in bio oils with uniform and highly negative sooting propensities. Finally, the scope of this work was expanded to a continental level through an exploratory study assessing the valorization potential of agricultural residues in Africa. An energy potential between 8 and 10 exajoules per year was established, representing potential GHG emission reductions of approximately 15 megatons of CO2 eq. compared to current residue management practices. In summary, this thesis demonstrates the effectiveness of densification and pyrolysis for lignocellulosic residue valorization and provides fundamental, applied, and prospective contributions to sustainable bioenergy pathways.

Date	5 Oct 2026
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Romain Lemaire (Supervisor), Josiane Nikiema (Co-supervisor) & Vincent Carré (Co-supervisor)