New Pressurized CCS System Could Cut Energy Penalty

Researchers at the Massachusetts Institute of Technology (MIT) looking into new power generation cycles have designed an innovative oxyfuel system that uses a pressurized coal combustor to capture and concentrate carbon dioxide emissions for direct injection into deep geological formations. The scientists say that the new approach reduces the energy penalty that all carbon-capture systems for power plants have by nearly 3%. But, even this small efficiency gain could enable technology to help make carbon capture and sequestration systems (CCS) practical and affordable, they say.

As Professor of Mechanical Engineering Ahmed Ghoniem and his team explain, any system for separating and concentrating carbon dioxide from a power plant reduces the efficiency of the plant by about a third. Ghoniem compared that process to "mixing salt and pepper," saying that mixing is easy, but separating them takes energy. "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" he said.

The team designed a coal plant combustion chamber that burns the fuel under pressure and uses a stream of pure oxygen instead of ordinary air (which is 79% nitrogen and 21% oxygen). The use of pure oxygen already eliminated more than three-quarters of the resulting flue gases, the team said.

In performing simulations and lab-scale tests of the new system, the researchers then demonstrated that the system recovers more thermal energy from flue gases because the elevated flue gas pressure raises the dew point and the available latent enthalpy (heat content) in flue gases. This high-pressure water-condensing flue gas thermal energy – recovery system reportedly eliminates the low-pressure steam bleeding, which is typically used in conventional system cycles. As a result, it enables the cycle to achieve an efficiency gain of nearly 3% compared with an unpressurized oxyfuel system.

In their tests, the researchers examined a flue gas purification and compression process that removed sulfur oxides and nitrogen oxides, and which used a low-temperature flash unit. The comparison was between combustors that operated at 1.1 bars versus 10 bars.

Ghoniem told the MIT news service in September that even though the process uses more energy at the beginning of the combustion cycle — because of the need to separate oxygen from air and pressurize it — the increased efficiency of the power cycle raises the net output of the plant. It also reduces the compression work needed to deliver CO2 at the requisite pressure for sequestration, as compared with unpressurized carbon-capture systems.

Pressurization of the combustion system also reduced the size of the components and hence the plant, which could "reduce the footprint of needed real estate, and potentially the price of components," he said. The efficiency gain of nearly 3% compared to an unpressurized system could probably be improved to a 10% to 15% gain with further research and development from current values, he said.

European power giant ENEL, which sponsored the research, is planning to build a pilot plant in Italy based on the technology if researchers manage to reach those gains. Ghoniem said, however, that much more study is needed in three specific areas: the operating conditions at which the different components work together for highest efficiency; component-level research to optimize the design of individual parts — especially the combustion chamber — of the new system; and process analysis to examine the details of the physics and chemistry involved.

The researchers’ findings are published in an August 2009 paper titled, "Analysis of Oxy-Fuel Combustion Power Cycle Using a Pressurized Coal Combustor." The pdf document is available online at http://web.mit.edu/mitei/docs/reports/hong-analysis.pdf.

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