Plastic membrane to bring down cost of carbon capture

Coal-fired power stations are acknowledged as major contributors to global carbon dioxide emissions. Research efforts and investments are underway aimed at developing techniques to reduce these emissions by capturing the carbon before it enters the atmosphere and sequestering it. However, such carbon capture and storage (CCS) systems are in an experimental stage and several hurdles remain. Capturing the CO2 is expensive and there are concerns about the safety of storing the gas in sites such as depleted oil and gas fields or saline aquifers. One of these problems has now been tackled by Australian PhD researcher Julianna Franco, who developed a cost-effective CO2 capture system based on the use of inexpensive plastic.

CCS systems can be applied to the use of biofuels in which case the energy generated in a power plant becomes carbon-negative. Contrary to quasi 'carbon-neutral' energy technologies (such as wind, solar or nuclear) which merely prevent the addition of new greenhouse gases in the future, Bio-energy with Carbon Storage (BECS) actually takes emissions of the past out of the atmosphere. It is because of this potential that we track developments in CCS technologies. Moreover, by testing CCS with biofuels instead of fossil fuels, risks can be reduced (previous post).

Several techniques currently exist to capture carbon dioxide. Broadly speaking, two categories can be distinguished: either the CO2 is captured before the fuel is combusted (pre-combustion capture), as would be the case in CO2 separation from biogas and in systems that first convert fuel into a synthesis gas; or the CO2 is captured from flue gas after the fuel has been used (post-combustion capture). Within the latter category several options are available, such as absorption with solvents, calcium cycle separation, cryogenic separation, membrane separation or adsorption by the fixation of CO2 molecules on a surface. Most of these are currently too expensive to make CCS systems commercially feasible.

The latest edition of Ecos Magazine, Australia´s most authoritative magazine on sustainability in the environment, industry and community published by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) reports [*.pdf] that University of Melbourne doctoral student Julianna Franco has developed a CO2 separation membrane made from inexpensive polypropylene. The development is set to bring down the costs of CO2 capture.

The plastic membrane can replace proposed CO2 capture systems based on expensive membrane materials such as Teflon. In this membrane gas absorption (MGA) system, the porous plastic acts as a semipermeable barrier, allowing CO2 gas on one side to come into direct contact with an aqueous solvent on the other, without the gas or liquid dispersing into each other:
energy :: sustainability :: biomass :: biofuels :: climate change :: coal :: carbon capture and storage :: carbon-negative :: bioenergy with carbon storage ::

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,’ says 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, too, 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.

Global interest in this research is demonstrated by the list of universities collaborating with the University of Melbourne: Canada’s Regina University, the University of Trondheim, Twente University in the Netherlands, and the University of Texas.

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. The trial is being funded by the Australian and Victorian Governments.

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 commercialscale carbon dioxide capture systems in Australia is 2015, although other sources put the date at 2020.

Photo: Researcher Julianna Franco working on a laboratory scale membrane-gas absorption unit for the separation of CO2 at the University of Melbourne. Courtesy: Cooperative Research Centre for Greenhouse Gas Technologies.

References:
Mary-Lou Considine: "Plastic membrane to bring down the cost of CO2 capture", ECOS Magazine, Issue 137, June-July 2007, pp. 32-32

Julianna Franco, Sandra Kentish, Jilska Perera, Geoff Stevens, "A Solution to Climate Change?" Chemistry in Australia, Issue 4, (2005), page 8–10.

Julianna Franco, Jilska Perera, Geoff Stevens, Sandra Kentish, "Membrane Gas Absorption using a Chemically Modified Polypropylene Membrane" [*.pdf/abstract], The University of Melbourne, Melbourne, Australia, The Cooperative Research Centre for Greenhouse Gas Technologies, Proceedings for the 8th International Conference on Greenhouse Gas Control Technology (GHGT-8), Trondheim, Norway, 19–22 June 2006.