Experimental study and optimization of a microwave plasma biomass pretreatment using highly viscous sodium carboxymethyl cellulose solutions

Abstract A recently proposed pulsed in-liquid microwave plasma pretreatment aiming for biomass disintegration and methane yield enhancement in subsequent anaerobic digestion was further studied and optimized experimentally. A water-based, transparent, highly viscous, sodium carboxylmethyl cellulose solution was used as the treatment model and enabled a systematic quantification of the ignitability of the discharge depending on operational, geometric and substrate parameters. Higher conductivity and initial viscosity along with more powerful pulses and a small microwave antenna cross section promoted the built-up of an initial vapor bubble and hence a fast, efficient and reliable plasma ignition. The observed large viscosity reduction of the model indicated its disintegration and was found to originate from the production of hydrogen peroxide and subsequent advanced oxidation processes. The release of tungsten, copper and zinc due to erosion of the microwave antenna led to additional catalytic effects. Subsequently, the geometry of the coaxial antenna and the shape of the MW pulses were optimized for the most effective and energy-efficient disintegration of the cellulose model, which aligned with the most efficient ignition and hydrogen peroxide production. The best-performing configuration compromised the newly introduced small tungsten coaxial antenna and microwave pulses of 2 kW and 4.6 ms power and length, respectively (10% duty cycle). These findings represent not only an improved understanding of the proposed plasma biomass pretreatment, but also directly led to an improved technical implementation. A sophisticated application on actual biomass substrates in future work was therefore prepared.