A little tinkering goes a long way

In some of the first results to find new ways of capturing carbon dioxide (CO₂) from coal-fired power plants, Rice University scientists have found that CO₂ can be removed more economically using "waste" heat -- low-grade steam that cannot be used to produce electricity.  In the process, about 10 percent of power loss can be avoided.

The find is significant because capturing CO₂ with conventional technology is an energy-intensive process that can consume as much as one-quarter of the high-pressure steam that plants use to produce electricity. The researchers hope to reduce the costs of CO₂ capture by creating an integrated reaction column that uses waste heat, engineered materials and optimized components.

Power plants fired by coal and natural gas account for about half of the CO₂ that humans add to the atmosphere each year; these power plants are prime candidates for new technology that captures CO₂ before it goes up in smoke. Each of these plants makes electricity by boiling water to create steam to run electric turbines. But not all steam is equal. Some steam has insufficient energy to run a turbine. This is often referred to as "waste" heat, although the term is something of misnomer because low-grade steam is often put to various uses around a plant.

Rice's new study found that in cases where waste is available, it may be used to capture CO₂. Employing waste heat is just one example of a number of ways that Rice's team is looking to improve upon a tried-and-true technology for CO₂ capture. The technology -- a two-phase chemical process -- has been used for decades to remove naturally occurring CO₂ from natural gas. In the first phase of the process, gas is piped upward through a vertical column while an ammonia-like liquid called amine flows down through the column. The liquid amine captures CO₂ and drains away while the purified natural gas bubbles out the top of the column.

In the second phase of the process, the CO₂-laden amine is recycled with heat, which drives off the CO₂. A major challenge in adapting two-phase amine processing for power plants is the amount of heat required to recycle the amine in the second phase of the process. This phenomenon is known as "parasitic" power loss, and it will drive up the cost of electricity by lowering the amount of electricity a plant can produce for sale.

The present research suggests that two elements of Rice's design -- optimized amine formulation and the use of waste heat -- can reduce parasitic power loss from about 35 percent to around 25 percent.