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Capturing Carbon with Membranes


By 2030, 80% of world energy will still be supplied by fossil fuels because the global energy demand during this period is expected to grow by 45%.

By Colin Scholes

Membrane technologies being developed in Australia hope to cut the cost of capturing industrial emissions of carbon dioxide.

It has been well-established that increasing carbon levels in the atmosphere are linked to global climate change. Therefore, to avoid the danger of catastrophic climate change there needs to be a global movement to reduce carbon emissions.

This presents a significant challenge because the world relies upon carbon-intensive industries to power the modern economy, such as electricity generation, fertiliser manufacture and metal smelting. Therefore, the scientific and engineering challenge of climate change is to develop technologies that can reduce carbon emissions cheaply.

Carbon Capture and Storage
One proposed strategy is carbon capture and storage (CCS), where carbon dioxide is separated from an industrial process before it can be released to the atmosphere, and the captured carbon dioxide is stored long-term in a safe manner. Importantly, this strategy allows existing industrial facilities to remain in operation. This is a big advantage, since currently 80% of Australia’s power generation comes from coal.

Converting our power generation facilities to renewable energy, such as wind and solar energy, is expected to cost billions and will take decades for the necessary infrastructure to be built. For example, the International Energy Agency estimates that 15% of the world’s energy will be supplied by renewables by 2030 (up from 3% in 1997), but 80% of world energy will still be supplied by fossil fuels because the global energy demand during this period is expected to grow by 45%.

Therefore, strategies that allow existing industries to remain in operation over the coming decades while preventing their carbon emissions from reaching the atmosphere are required. This is why CCS is a strategy that needs to be investigated and developed.

The biggest obstacle to implementing CCS is cost, and developing cheap carbon capture technologies is of paramount importance since the capturing process is estimated to be as much as 85% of the total cost of CCS. This is why the CO2CRC is leading the world in operating two demonstrations of carbon capture technologies: the H3 Capture Project and the Mulgrave Capture Project.

There are a range of potential technologies for capturing carbon emissions. Currently, reversible solvent absorption is the most widely applied. This uses a reversible chemical reaction between carbon dioxide and a salt or amine solution. Carbon dioxide-rich flue gas is bubbled through the solution, and it reacts with the solution and removes it from the flue gas. The carbon dioxide-loaded solution is then pumped to a separate process, where it is heated to near boiling and the reverse reaction occurs, with the carbon dioxide gas separated from the solution. This produces high purity carbon dioxide gas that can then be transported to the storage site while regenerating the solution so that it can be recycled. This process, however, is very energy-intensive, and therefore costly, because it requires heating of the solution to almost boiling conditions to reverse the reaction.

Other examples of potential carbon capture technologies that are still in development are adsorption systems, which rely on carbon dioxide reacting with solid inorganic particles, as well as cryogenics, where the waste gas is cooled to allow carbon dioxide hydrate ice to form.

Using Membranes
Membrane technology is another carbon capture technology that has the potential to significantly reduce the cost of capture. A simple membrane-based separation process has advantages over traditional solvent-based technology due to lower energy needs – there is no heating or cooling of solutions.

The membrane process works by relying on a semipermeable material – often a polymer or plastic – that allows carbon dioxide to pass easily through while preventing other gases, such as nitrogen and oxygen, from doing likewise. This removes carbon dioxide from a gas stream by concentrating it on the other side of the membrane material. Once the carbon dioxide is concentrated, it can then easily be transported to the storage process.

One of the best examples of membrane separation technology currently in use is desalination of salt water, where the membranes used are permeable to water but not salt, allowing the generation of fresh water through reverse osmosis.

A range of polymers can be used to manufacture carbon dioxide membranes through an approach known as solution diffusion. Here the membrane film has no pores or holes, and carbon dioxide passes through the material by being very soluble in the polymer itself while other gases, such as oxygen and nitrogen, are not. This is achieved by taking advantage of intermolecular interactions that carbon dioxide can undergo while other gases, especially diatomic gases such as oxygen and nitrogen, cannot. This means that the polymer can concentrate carbon dioxide throughout its structure and therefore allow the carbon dioxide to transport freely through the material. The overall driving force for the transport of carbon dioxide through the material is the difference in pressure across the membrane.

It is surprising the number of polymers that can act as a carbon dioxide membrane. Even simple plastics used in food packaging have some ability, though only weakly. Polymers that have excellent performance are defined by two criteria: a high carbon dioxide flux and high carbon dioxide selectivity compared with other gases.

Given the nature of polymer chemistry, considerable research is currently focused on improving the performance of these types of polymers. This is achieved through the generation of copolymers and polymer blends, adding extra materials such as nanoparticles to the polymer system, and altering membrane formation techniques and annealing temperature. This research has developed a range of potential polymeric systems that can be fabricated as excellent membranes for carbon capture.

However, creating a high-performing membrane in the laboratory is only half the story. The membrane must be practical for use in industrial processes, which have considerably harsher environments than those tested in laboratories. For instance, a range of other gases and chemicals are often present in industry at low concentrations, such as sulfur oxides, nitric oxides, hydrogen sulfide, ammonia and heavy hydrocarbons such as hexane. Furthermore, the process gas is generally saturated with water that, combined with the other chemicals present, makes for a corrosive environment.

This can have a dramatic implication on membrane performance and rapidly corrode the polymer, leading to membrane rupture. Hence the CO2CRC operates two membrane-based carbon capture facilities to determine the performance of membranes in real processes.

Demonstration Plants
The CO2CRC’s H3 Capture Project separates carbon dioxide from flue gas at a coal-fired power station in Victoria’s Latrobe Valley. The membrane plant is designed to separate out 15 tonnes of carbon dioxide per annum, which is the equivalent of capturing the emissions of one Australian per year.

The pilot plant is part of the bigger H3 Capture Project, which is evaluating the performance of three carbon capture technologies in a power plant environment. The objective for the membrane pilot plant is to separate carbon dioxide from flue gas, which is about 10% carbon dioxide and the remainder nitrogen, and to verify that membrane separation performance can withstand the flue gas environment. Of particular interest is the effect that water in the flue gas will have on the membrane, given that sulfur oxides and nitric oxides are also present (from the coal combustion process), giving rise to acidic conditions. The plant was commissioned in July 2009 and has been successfully capturing carbon dioxide intermittently since then.

The other capture demonstration is the CO2CRC Mulgrave Capture Project, which is based on the separation of carbon dioxide from synthetic gas (syngas) generated from the gasification of coal. Syngas is a mixture of carbon dioxide, carbon monoxide and hydrogen. It is an important industrial gas that is used for a range of processes, such as hydrocarbon synthesis and the production of fertiliser from ammonia synthesis, as well as possibly power generation through the burning of hydrogen.

The membrane separation facility operates as part of a range of carbon capture technologies in the Mulgrave Project. It is designed to separate 5.6 kg of carbon dioxide per day from syngas, which is about 13% carbon dioxide, 12% carbon monoxide, 12% hydrogen and the remainder nitrogen.

The main objective of this facility is to separate carbon dioxide from hydrogen. This is considerably harder than separating from nitrogen, given hydrogen’s small size, but it can be achieved by basing the membrane material on rubbery polymers such as polydimethyl siloxane and polyethylene glycol. The small scale of the project makes the facility very adaptable in terms of process configuration. This has allowed a range of membrane separation designs to be trialled to maximise separation efficiency.

The next stage in applying membranes to carbon capture will be dictated by the continual operation of both facilities. Successful outcomes over the next year will demonstrate the potential of membranes in carbon capture, and allow the efficiency of the technology to be determined. Larger-scale demonstration facilities could then be designed and constructed in the near future to prove the viability of this technology to Australian industry while capturing carbon in significant quantities over a longer period. Success see membrane technology used in carbon capture mature over the next 5–10 years, in keeping with international targets for carbon emission reductions.

Many carbon capture technologies, including membranes, are currently ready to be applied in some form to reduce the emissions of many industrial processes. Currently, the biggest obstacle to the uptake of membrane technology, along with many of the other approaches to reduce carbon emissions, is that there is very little incentive for industry to actually cut emissions. Only recently have penalties for emitting carbon to the atmosphere been considered, and therefore there is no economic incentive for industries to reduce carbon emissions.

This is the reason for the current global debate around putting a price on carbon. Hence, any commitment on reducing carbon emissions must ultimately come as a result of everyday people demanding action on climate change.

Colin Scholes is a Research Fellow at the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC), the University of Melbourne.