Producing biogas from the bacterial breakdown of biomass seems a relatively simple idea: just let the bacteria do their thing on a pile of plant or animal material.
In reality, however, the complex composition of biomass presents challenges; not the least is that lignocellulose in plants (the woody bits) can be hard for bacteria to digest.
Pre-treatment using chemical, physical and/or mechanical processes is possible, but these can be expensive, inefficient or far from environmentally sound.
With this in mind, German researchers are testing plasma formation in biomass with, they say, promising results.
With the help of microwave pulses, they have found that plasma induced into polymer solutions helps break down the thick material, lowering the viscosity and possibly breaking polymer chains.
“The plasma can be seen as a reactive gas, which contains populations of particles that contain several electron volts of kinetic energy,” says Bruno Honnorat from Leibniz Institute of Plasma Science and Technology, co-author of a paper in the journal AIP Advances.
“This energy can be used to break the bond of the chemicals and break the bonds of molecules with which they interact.
“The most surprising thing was to be able to obtain plasma discharge conditions in a moving liquid. The presence of a flow considerably complicates the situation compared to all the other experimental setups studied in the literature.”
The work involves creating a reactor in which two-kilowatt microwave pulses injected into a moving liquid model induce plasma formation within one millisecond.
The totality of the microwave power is concentrated to a small cavity, containing less than one millilitre of liquid, which is heated, vaporised, and finally ignited, forming an expanding plasma bubble.
The plasma-liquid interaction forms reactive species, including oxidizing agents, such as hydroxyl radicals and hydrogen peroxides, that help break down the biomass and decrease the viscosity, or resistance to flow, of the biomass material.
The process will be now further tested at full scale in a biogas plant, and the research will continue.