An industrial power plant at sunset

Part 1: Carbon Dioxide Capture – Post Combustion

Greenhouse gases and global warming have, over time, become accepted as the reason for Climate Change. Environmental scientists have concluded that greenhouse gases (GHG) are a major contributor to global warming. Due to their low cost, availability, existing reliable technology for energy production, and energy density, fossil fuels currently supply over 85% of the United States’ energy needs and a similar percentage of the energy used worldwide 1. Unfortunately, CO2 is a significant contributor to greenhouse gases, but it is becoming a possible economic resource for use in industry and energy production sectors.

The Department of Energy’s goal is to have the necessary technology ready for large scale field testing, should it become necessary to impose mandatory limits on CO2 emissions. None of the currently available CO2 capture processes are economically feasible on a national implementation scale to capture CO2 for sequestration since they consume large amounts of parasitic power and significantly increase the cost of electricity.

One approach that holds great promise for reducing GHG emissions is carbon capture and sequestration (CCS). Under this concept, CO2 is captured from large point sources, such as power plants, and injected into geologic formations, such as depleted oil and gas fields, saline formations, and exhausted coal mines 2 . The CO2 is pumped into these voids in the earth and would stay there for potentially thousands of years. The problem with CO2 capture technologies is how to capture the CO2 from the burning of fossil fuels in the first place. Three means are being studied; Post Combustion CO2 Capture, Pre-Combustion CO2 Capture, and Oxygen Combustion.

Post Combustion CO2 Capture:

Post Combustion CO2 Capture seems the most likely method of reducing GHG formation from burning fossil fuels. Post-combustion capture involves the removal of CO2 from the flue gas produced by combustion. Many technologies are being studied, including Amine Based CO2 Capture, Carbonate conversion, Membranes, and Enzyme Based Systems.

Amine based CO2 capture uses Amines that react with CO2 to form water soluble compounds. Because of this compound formation, amines are able to capture CO2 from streams with a low CO2 partial pressure, but capacity is equilibrium limited. The Amine must be heated to regenerate the amine and release the CO2. Thus, amine-based systems are able to recover CO2 from the flue gas of conventional pulverized coal (PC) fired power plants, however only at a significant cost and efficiency penalty.3

Carbonate systems are based on the ability of a soluble carbonate to react with CO2 to form a bicarbonate, which when heated releases CO2 and reverts to a carbonate. The carbonate then acts as a capture mechanism that is then heated to release pure CO2 thereby regenerating the carbonate. A major advantage of carbonates over amine-based systems is the significantly lower energy required for regeneration4.

There are a variety of options for using membranes to recover CO2 from flue gas. In one concept, flue gas is passed through a bundle of membrane tubes, while an amine solution flows through the shell side of the bundle. CO2 passing through the membrane is absorbed in the amine, while impurities are blocked from the amine, thus decreasing the loss of amine as a result of stable salt formation. The amine would then be regenerated for further use and releasing the CO2 for capture.

The University of New Mexico 5  is developing another concept using inorganic membranes. Researchers have shown the ability to prepare a silica membranes that can selectively separate CO2 from CH4 and are developing a microporous inorganic silica membrane containing amine functional groups for the separation of CO2 from flue gas.

Biologically based capture systems are another potential avenue for improvement in CO2 capture technology. These systems are based upon naturally occurring reactions of CO2 in living organisms. One of these possibilities is the use of enzymes. An enzyme-based system, which achieves CO2 capture and release by mimicking the mechanism of the mammalian respiratory system, is under development by Carbozyme6 . The mechanism uses carbonic anhydrase to capture the CO2 in the flue gas just like our body does. The human body can capture up to 2 lbs. of CO2 daily and the Carbozyme System would be scaled up dramatically to take capture even more. Carbozyme created a synthetic version of carbonic anhydrase, coated millions of microscale porous tubes with it, and is doing lab tests. As flue gases pass through the tubes, the enzyme captures the CO2 from it, turns it into bicarbonate and back, then it isolates it, getting ready for pumping and storing it into voids for carbon dioxide sequestration.