One of the main bottlenecks to deploying large-scale carbon dioxide capture and storage (CCS) in power plants is the energy required to separate the CO₂ from flue gas. For example, near-term CCS technology applied to coal-fired power plants is projected to reduce the net output of the plant by some 30% and to increase the cost of electricity by 60–80%. Developing capture materials and processes that reduce the parasitic energy imposed by CCS is therefore an important area of research.

—Lin et al.

The research by scientists at Rice University; the University of California, Berkeley; Lawrence Berkeley National Laboratory (LBNL); and the Electric Power Research Institute (EPRI) was published in the journal Nature Materials.

Although no commercial power plants currently capture carbon dioxide on a large scale, a few small-scale and pilot plants do, using amine scrubbing: funneling emissions through a bath of nitrogen-based amines, which capture carbon dioxide from the flue gases. The amines are then boiled to release the CO₂. Additional energy is required to compress the carbon dioxide so that it can be pumped underground.

The energy needed for this process decreases the amount that can go into making electricity. Calculations show that for a coal-fired power plant, that could amount to approximately 30% of total energy generated.

Solid materials should be inherently more energy-efficient than amine scrubbing, because the CO₂ can be driven off at lower temperatures. But materials differ significantly in how tightly they grab CO₂ and how easily they release it.

One of the 50 best zeolite structures for capturing carbon dioxide. In the model, the red balls are oxygen, the tan balls are silicon. The blue-green area is where carbon dioxide prefers to nestle when it adsorbs. (Berend Smit laboratory, UC Berkeley) Click to enlarge.

Zeolites are common minerals made mostly of silicon and oxygen. About 40 of the highly porous materials exist in nature, and there are about 160 man-made types. The pore sizes and shapes vary depending upon how the silicon and oxygen atoms are arranged. The pores act like tiny reaction vessels that capture, sort and spur chemical reactions of various kinds, depending upon the size and shape of the pores. The chemical industry uses zeolites to refine gasoline and to make laundry detergent and many other products.

In 2007, Deem at Rice University and colleagues calculated millions of atomic formulations for zeolites, and they have continued to add information to the resulting catalog, which contains about 4 million zeolite structures.

In the new study, the zeolite database was examined with a new computer model designed to identify candidates for CO₂ capture. The new model was created by a team led by corresponding author Berend Smit, UC Berkeley’s Chancellor’s Professor in the departments of chemical and biomolecular engineering and of chemistry and a faculty senior scientist at LBNL. Smit and his UC Berkeley group worked with study co-author Abhoyjit Bhown, a technical executive at EPRI, to establish the best criteria for a good carbon capture material. Focusing on the energy costs of capture, release and compression, they created a formula to calculate the energy consumption for any materials in the zeolite database.

What is unique about this model is that, for the first time, we are able to guide the direction for materials research and say, ‘here are the properties we want, even if we don’t know what the ultimate material will look like’. Before, people were trying to figure out what materials they should shoot for, and that question was unanswered until now.

—Abhoyjit Bhown

Running the calculations to compare the CO₂-capture abilities of each zeolite would have taken approximately five years with standard central processing units (CPUs), so Smit and his colleagues at UC-Berkeley and LBNL created a new way to run the calculations on graphics processing units (GPUs). Deem said the GPU technique cut the compute time to about one month, which made the project feasible.

Our database of carbon capture materials is going to be coupled to a model of a full plant design, so if we have a new material, we can immediately see whether this material makes sense for an actual design.

—Berend Smit

Study co-authors include graduate students Li-Chiang Lin and Joseph Swisher, both of UC Berkeley; Adam Berger of the EPRI; Richard Martin, Chris Rycroft and Maciej Haranczyk, all of LBNL’s Computational Research Division; and postdoctoral fellows Jihan Kim and Kuldeep Jariwala of LBNL’s Materials Science Division. This research was supported by the Department of Energy, the Advanced Research Projects Agency–Energy and EPRI’s Office of Technology Innovation.

Resources

• Li-Chiang Lin, Adam H. Berger, Richard L. Martin, Jihan Kim, Joseph A. Swisher, Kuldeep Jariwala, Chris H. Rycroft, Abhoyjit S. Bhown, Michael W. Deem, Maciej Haranczyk & Berend Smit (2012) In silico screening of carbon-capture materials. Nature Materials doi: 10.1038/nmat3336