Design of a Three-Stage Filtration System to Remove Contaminant Sulphur Compounds from a CO2-Enriched Gas Supply for Use in Long-Term Tree Growth Experiments

Abstract

The design of a three-stage filtration system to remove contaminating hydrogen sulphide from a supply of carbon dioxide for long-term tree growth experiments is described. The system is designed to treat a gas mix of 75% CO₂ in air at a flow rate of 30 m³ h⁻¹ originating from biogas at a waste water treatment plant. The concentration of H₂S in the raw gas is 2500 µl l⁻¹ and this is reduced to less than 1 µl l⁻¹ by the filtration process. The three filtration stages consist of a recirculating liquid system using a self-regenerating solution of iron chelate, a dry iron oxide box, and drums containing activated carbon. Regular, low intensity maintenance is necessary to remove the precipitated sulphur compounds from the liquid system. The iron oxide and activated carbon need to be changed at three monthly and monthly intervals, respectively. The spent iron oxide is discarded, while the activated carbon is able to be regenerated using caustic soda solution. After filtering, the gas is diluted with ambient air in large open-top chambers surrounding the trees. The air in the chambers has a concentration of CO₂ of approximately 650 µl l⁻¹ with the concentration of H₂S being less than 0.001 µl l⁻¹, which is acceptable for tree growth studies. The system is reliable and has operated continuously for 3 years. This provides a low cost option for the long-term supply of CO₂ to experimental field sites.

Keywords:

• Carbon dioxide
• purification
• hydrogen sulphide
• regeneration
• biogas