Enhancing small-scale biomass gasification through steam and oxygen injection: Experimental and numerical analysis by Arnaud Rouanet

Lignocellulosic biomass gasification is a promising sustainable alternative to fossil fuels in several applications, such as high-temperature industrial heat for industries or decentralized and flexible Combined Heat and Power (CHP). Gasification converts solid biomass resources such as forestry residues or wood waste into a more versatile and CO2-neutral gaseous fuel called syngas.

Syngas produced by small-scale downdraft gasification is affected by two main challenges: it has a low energy density and contains residual tar that can harmfully condense in downstream processes. Steam and oxygen injection is seen as a way to address these challenges and enhance the quality of syngas. This thesis combines numerical models and experimental campaigns to investigate their effects on two-stage downdraft gasification.

Experiments on a pilot two-stage downdraft gasifier have shown that replacing secondary air with oxygen and steam enhances the syngas Lower Heating Values (LHV) by 55%, from 4.4 to 6.8 MJ∕Nm3. Steam mainly plays the role of temperature damper to avoid excessive process temperatures, but adversely affects the LHV. Steam also shifts the syngas composition to a higher H2/CO ratio.

This higher syngas LHV translates into better engine volumetric efficiency in CHP applications and a higher flame temperature for industrial burners. Although oxygen and steam enhance the gasification efficiency, the energy required for their production more than offsets the gains, resulting in a neutral to negative net energy balance. Process integration can limit this unfavorable effect, mainly through heat recovery steam generation from the syngas sensible enthalpy.

With air, steam reduces tar production, but combining oxygen and steam yields higher amounts of Class III and IV tars. This phenomenon is attributed to a less efficient mixing of oxygen and volatiles, which a modification of the injection nozzles could solve. The amount of tar remains very low compared to most other gasification technologies.

Extending the use of oxygen and steam to the primary stage first requires a better understanding of the propagation dynamics of the "smoldering" front through the biomass bed. A numerical model of the pyrolysis zone has been built by coupling a CFD model of biomass combustion with a single-particle pyrolysis model to consider the effect of particle thermal thickness. Despite promising preliminary results, the model still requires several improvements.Future research should aim to entirely replace air with oxygen and steam, using numerical simulations to identify adequate operating conditions, followed by experimental campaigns to validate their effect. In the long term, expanding the CFD model to the complete two-stage gasifier would produce a valuable tool to simulate operating conditions and support experimental results interpretation.

Jury members :

• Prof. Hervé Jeanmart (UCLouvain, Belgium), supervisor
• Prof.  Sandra Soares-Frazao (UCLouvain, Belgium), chairperson
• Prof.  Juray De Wilde (UCLouvain, Belgium)
• Prof. Julien Blondeau (VUB, Belgium)
• Prof. Frederik Ronsse (UGent, Belgium)
• Prof. Jacobo Porteiro Fresco (Universidade de Vigo, Spain)
• Dr. Yves Ryckmans (ENGIE Laborelec, Belgium)

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