New carbon capture system could cut today's 'energy penalty'

A new system for capturing carbon dioxide emissions from coal-fired power plants could reduce the “energy penalty” that reduces the efficiency of other carbon capture methods, according to new research from the Massachusetts Institute of Technology (MIT).

The MIT strategy, called “pressurised oxy-fuel combustion,” uses more energy at the beginning of the combustion cycle, but improves plant efficiency and reduces the energy needed to pressurise captured carbon dioxide at the end of the cycle.

Captured in liquid form, the carbon dioxide can then be injected into geological formations that are deep enough to prevent the gas from escaping back into the atmosphere.

Finding a cost- and energy-efficient way to capture and store carbon emissions (known as “CCS,” for “carbon capture and storage”) is considered vital to fighting climate change while continuing to use fossil fuels. More than 90 per cent of the world’s energy currently comes from fossil fuels, and carbon dioxide emissions related to that use are projected to increase by more than 50 per cent by 2030.

“This is the first step,” said Ahmed Ghoniem, a mechanical engineer and leader of the MIT team analysing the new technology. “Before you sequester, you have to concentrate and pressurise. You have to redesign the power plant so that it produces a pure stream of pressurised liquid carbon dioxide, to make it sequestration-ready.”

The MIT team described its approach in the August edition of the journal Energy. ENEL, an Italian energy company that sponsored the research, plans to build a pilot plant in Italy using the technology in the next few years.

Currently, any system for separating and concentrating the carbon dioxide from a power plant reduces the efficiency of the plant by about a third. That means more fuel is needed to provide the same amount of electricity. Finding ways to minimise that efficiency loss is believed to be one key to making carbon-sequestration systems commercially viable.

There will always be some energy penalty for carbon capture, because it takes energy to separate gases that are mixed together in the form of emissions.

“Mixing salt and pepper is very easy, but separating them takes energy,” said Ghoniem. “Nobody in their right mind will jump into this and do it unless we can reduce the energy penalty and the extra cost, and only if it is mandated to reduce CO2 emissions.”

Other groups have been looking into oxy-fuel combustion, in which pure oxygen is fed into the combustion chamber to produce a cleaner and more concentrated emissions stream. The MIT studies approach adds one more element: putting the whole combustion chamber under pressure, which results in a more concentrated, pressurised emissions output.

Even though this process uses more energy at the beginning of the combustion cycle — because of the need to separate oxygen from air and pressurise it — the increased efficiency of the power cycle raises the net output of the plant and reduces the compression work needed to deliver CO2 at the requisite state for sequestration.

The pressurisation of the combustion system also reduces the size of the components and, hence, the plant, which could “reduce the footprint of needed real estate, and potentially the price of components,” Ghoniem said.

Compared to an unpressurised system, the MIT approach could lead to an overall improvement of about 3 percent in net efficiency. With further research and development, Ghoniem said, that could probably be improved to about a 10- to 15-per cent net gain from current values.

Still, a lot of problems still need to be resolved before CCS technology can be commercially viaable. Ghoniem said the three areas that need study most are systems’ integration to determine the operating conditions at which the different components work together for highest efficiency; component-level research to optimise of the design of individual parts of the new system, especially the combustion chamber; and process analysis to examine the details of the physics and chemistry involved.