24 September 2009

Concentrating coal plant emissions

There is a new approach to eliminate carbon dioxide (CO₂) emissions at coal-burning power plants.

The system, pressurized oxy-fuel combustion, provides a way of separating all of the CO₂ emissions produced by the burning of coal, in the form of a concentrated, pressurized liquid stream, said researchers at MIT. This will allow for CO₂ sequestration where the liquid CO₂ stream can inject into geological formations deep enough to prevent their escape into the atmosphere.

Finding a practical way to sequester carbon emissions is critical in the effort to mitigate climate change while continuing to use fossil fuels, which currently account for more than 80% of energy production in the U.S. and more than 90% worldwide. Projections call for CO₂ emissions from fossil fuels to rise by more than 50% worldwide by 2030.

It might seem odd to reduce the carbon footprint of a coal plant by making its emissions into a more concentrated stream of CO₂.

However, “this is the first step,” said Ahmed Ghoniem, the Ronald C. Crane Professor of Mechanical Engineering and leader of the MIT team analyzing this new technology. “Before you sequester, you have to concentrate and pressurize” the greenhouse gases. “You have to redesign the power plant so that it produces a pure stream of pressurized liquid carbon dioxide, to make it sequestration ready.”

There are various approaches to carbon capture and sequestration being developed and tested, and the oxy-fuel combustion system “is one of the technologies that should be looked at,” said Barbara Freese, lead author of a report on coal power by the environmental group Union of Concerned Scientists.

Ghoniem said of the approaches to oxy-fuel combustion, he and his MIT colleagues are the only academic team examining a pressurized combustion system for carbon dioxide capture.

Ghoniem said any system for separating and concentrating the CO₂ from a power plant reduces the efficiency of the plant by about a third. That means it takes more fuel to provide the same amount of electricity. Therefore, finding ways to minimize that loss of efficiency is key to making carbon sequestration systems commercially viable.

There will always be some energy penalty to capture-enabled systems because it requires some energy to separate gases mixed together, such as separating CO₂ from the combustion gases emerging from an air-based combustion chamber or oxygen from air for oxy-fuel combustion. As an analogy, “mixing salt and pepper is very easy, but separating them takes energy,” he said. “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. And that is what the new process should do.

The focus of their studies is a system that puts the whole combustion chamber under pressure, which results in a more concentrated, pressurized 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 pressurize 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, as compared to the unpressurized carbon-capture systems; in other words, the overall energy penalty comes down, Ghoniem said.

“You have to deliver carbon dioxide at high pressure for sequestration,” Ghoniem said. The system simply introduces some pressurization earlier in the process, so the output stream requires less compression at the end of the process while extracting more energy from the combustion gases.

The pressurization 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,” he said. It could lead to an overall improvement of 3% in net efficiency compared to an unpressurized system; and with further research and development, this can probably improve to about a 10 to 15% net gain, he said.

For related information, go to www.isa.org/manufacturing_automation.