CO2 capture

University of Melbourne PhD student Julianna Franco has developed a cost-effective CO2 capture system based on the use of inexpensive plastic.

In her membrane gas absorption (MGA) system, the porous plastic acts as a semi permeable barrier, allowing CO2 gas on one side to come into contact with an aqueous solvent on the other, without the gas or liquid dispersing into each other.

MGAs are commonly used to remove gases from, or dissolve them into, water. For an MGA to be effective, however, the membrane must be water-repellent on one side to prevent the water from passing through the pores into the gaseous side of the membrane.

According to Franco’s supervisor, Professor Geoff Stevens, past research on the use of polypropylene as a membrane for CO2 capture concluded that it was unsuitable for MGA use. The plastic’s pores were observed to ‘wet’ in the presence of the aqueous solvent used to absorb CO2
from the gaseous phase, allowing the two phases to mix.

However, Franco has now modified the surface properties of the polypropylene to make it as water-repellent as Teflon. This allows the CO2 to selectively pass through the membrane and be absorbed on the other side by a widely available solvent (20–30% methylethanolamine dissolved in water).

The membrane can be deployed in the form of hollow fibre units that provide an order of magnitude more surface area than those available in conventional CO2 capture columns.

‘MGA units can separate carbon dioxide using three to four times less space than processing towers, making carbon dioxide capture more efficient and economical,’ said Franco.

Franco’s research follows on from earlier research that resulted in the construction of a pilot-scale membrane gas absorption plant – incorporating Teflon as the membrane material – for separation of CO2 from natural gas at Kårstø, Norway.

Australia has natural gas reserves with high CO2 levels, such as those at the Gorgon gas field on the north-west coast of Australia. According to Stevens, a polypropylene MGA system would make new natural gas fields with high CO2 content more economically – and environmentally – viable.

However, it’s still early days. Stevens says the polypropylene carbon capture system is due to be tested next year at a pilot plant that will process 25 tonnes of CO2 per day. The pilot plant is being built at Hazelwood, one of Victoria’s oldest – and its most greenhouse-polluting – brown coal-fired power stations.

Depending on the outcome of that trial – in particular, how the economics of this technology stack up against competing technologies – the most optimistic date for the full deployment of commercial scale carbon dioxide capture systems in Australia is 2015.