Mark YOUNG1, John HOLBREY1, Sophie FOURMENTIN2, Leila MOURA1
1Queen's University Belfast, Belfast, United Kingdom
2University Littoral Cote d'Opale, Dunkerque, France
Biogas is a carbon neutral energy source produced from the anaerobic digestion of organic waste. Its major components are methane (CH4) and carbon dioxide (CO2) with other impurities present in smaller quantities such as volatile organic compounds (VOCs). Upgrading of biogas to biomethane can be carried by pressure and temperature swing adsorption, liquid scrubbing, membrane separation and cryogenic methods. However, in general these methods involve large facilities with multi-step, energy intensive processes that often incorporate hazardous materials [1]. Alternative low-energy purification processes would make biogas a significantly more attractive as an energy source of methane for small scale widespread application. Changing form multiple steps to a single technology for multiple contaminants would lead to a process that could be implemented on a much smaller geographical footprint ideal for smaller agricultural purposes.
Low melting mixtures (LMMs) offer alternatives to traditional biogas upgrading sorbents such as alkanolamines, however lower CO2 capacities coupled with high viscosities of physisorbant LMMs has limited their application scope. Here we report the study of hydrophobic LMMs based on trioctylphosphine oxide (TOPO) [2,3] that couple lower viscosities and a high tolerance to environmental water with high CO2 capacity. Measurements using headspace GC have also shown that these are effective solvents for the removal of several different classes of VOC commonly found in biogas which could enable them to be implemented as a “one pot” upgrading method.
To screen the performances of these materials, we have developed a novel and fast screening method for gas sorbents, allowing for multiple materials to be screened for pure and mixed gas uptake at pressures below 3.5 bar, simultaneously. Promising materials were then more accurately tested for gas capacity using an isochoric saturation method based on PvT measurements at various temperatures. This has shown that the best of these materials have CO2 capacities comparable to that of highly fluorinated ILs such as [BMIM][NTf2]. Experiments carried out utilizing both single and CO2/CH4 mixed gasses have also shown that these materials have comparable selectivities for CO2 over CH4 when compared to fluorinated ILs. Partition coefficients of VOCs in the same LMMs previously tested for gas sorption are also lower than that of many standard sorbents. These measured properties along with the relatively low viscosity make these materials seem like a viable new technology for biogas upgrading in the future.
REFERENCES
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