Removing CO2 from the Source

We have just experienced the warmest year on record; a year that has seen significant and devastating storms in the Gulf of Mexico. Around the world political and business leaders are now acknowledging the important contribution of greenhouse gases (GHG) to global climate change. Insurers have carried out cost analyses of the liabilities associated with global climate change. These liabilities have increased 15-fold and are expected to increase further unless something significant is done.

Releases of carbon dioxide (CO₂), methane (CH₄) and nitric oxide (NO₂) are the three primary contributors to GHG increases. Because of the vast quantity released, CO₂ is the most potent greenhouse gas driver. On a million metric ton of carbon equivalent basis, CO₂ accounts for 82% of the GHGs released. CO₂ comes from four sources – industrial (~29% in the U.S.), transportation, residential and commercial. On a fuel basis 21% is from burning natural gas, 42% from petroleum and 37% from coal. Electricity production accounts for ~79% of the industrial fraction. Because industrial point sources emit such huge quantities and are so readily identified they are the first sites for regulation. Increases in population and standard of living drive demand for energy. Most of the energy needed comes from burning hydrocarbons, especially coal. The largest economies have hundreds of years of coal available. Thus, despite the growth in renewables changing the course of this ship will be difficult.

The dumping of CO₂ into the atmosphere started to increase at the beginning of the Age of Industrialization, according to the Carbon Dioxide Information Analysis Center (CDIAC) run by the US Government’s Oak Ridge National Laboratory (Fig. 1).

Figure 1. Atmospheric CO2 increases since 1750 (blue) and the human contribution since 1850

At Mauna Loa, the site of longest continuous measurement of CO₂ in ambient air, the percentage of CO₂ in dry air was 0.0316% in 1959 and 0.0377% in 2004. This increase in CO₂ is not a direct threat to the air we breathe – dry air is just under 21% O2 and approximately 79% N2. The threat to humans is that the increase in CO₂ in the atmosphere is altering the world atmosphere and the geochemistry of oceans. CO₂ dissolves in the oceans resulting in an acidification that can have adverse effects on sea life. Unlike many other gases released with the burning of hydrocarbons CO₂ remains in the atmosphere for very long periods – hundreds of years (http://en.wikipedia.org/wiki/Greenhouse_gas). The increase in atmospheric CO₂ reveals that the production/absorption balance of CO₂ that has been operative over the last many centuries has broken down. Something needs to be done to decrease the source of CO₂, and to remove it from the global environment if we do not want to risk the consequences of global climate change.

The human cost of increasing atmospheric CO₂ cannot be overstated. The increase in tropical storms during the Northern summer has been devastating to human life and property. The threat of melting ice shelves to ocean water levels is ongoing. The risks include substantial redistribution and reduction in rainfall, more and greater storms, loss of low lying lands and displacement of persons living there. The possibility of a new ice age in western Europe even exists. The observation that the ambient air temperature has been increasing since industrialization is well documented; a quick glance at data from the Oak Ridge National Laboratory’s CDIAC showed that 2 areas close to my office in central New Jersey, in Moorestown and Atlantic City State Marina, have both had increases in mean ambient temperature of over 5.5°C (10°F) since the mid 1800s.

The question asked for decades is whether increased CO₂ is a cause of global warming: this was answered in the affirmative by a careful study reported in June 2005 in the weekly publication Science from the NASA-funded Goddard Institute at Columbia University. The authors, led by Dr. J. Hansen, additionally reported the information that the earth is absorbing more energy (0.85 ± 0.15 watts per square meter) from the sun that it emits to space, i.e., that the earth is warming.

Three strategies are under consideration to alleviate the effects of the continued use of hydrocarbons, particularly, coal, as primary energy sources – pre-combustion, enhanced combustion and post-combustion. Pre-combustion uses energy to generate hydrogen from water while capturing the CO₂, SOx, NOx and mercury. While attractive in principal this approach is expensive and has not yet attained the stability needed for routine use. Enhanced combustion relies on an air separation unit to produce oxygen. The oxygen is then combined with the recycled exhaust gases so that the new fuel charge is burned in an oxygen:CO₂ mixture. The result is more efficient combustion, an absence of NOx, and easy capture of CO₂. However, SOx, mercury and particle scrubbing are still needed. Post-combustion uses one of several methods to clean up stack gases from burning hydrocarbons. Pulverized coal (PC) is the best established and currently most robust method for producing electricity. Between plants currently in place and those under construction or in design PC will be with us for at least the next 50 years.

The concept now most popular for dealing with post-combustion CO₂ is to capture it (enrich it from the flue gas feed of <20% to >92%), pressurize it to super critical conditions and pipeline it to a final destination and then store it in geologic formations, i.e., active or abandoned oil fields, gas fields, or coal mines. In the case of active sites, additional energy could be derived by producing more oil or gas (CO₂ adsorbs to coal freeing methane). The remaining 80%+ of CO₂ would be stored in deep saline aquifers. Capture is recognized as the most difficult and most expensive of these tasks. It is expected to account for ¾ of the total cost.

In theory, post-combustion CO₂ could be captured by any of the methods classed operationally as adsorption, absorption, reaction, membranes or cryogenic separation. Many alternate technologies in these classes have been tested under the US Department of Energy (US DOE) Carbon Capture Program. They were all subject to evaluation under the same set of operating conditions and were categorized on the basis of their economic impact. From an economic perspective the object is to minimize the energy penalty associated with capture, expressed in plant derating or cost of CO₂ avoidance. The US DOE would like to see CO₂ capture from coal flue gas remove 90% of the CO₂ in the feed stream, achieve 95% enrichment, and accomplish this with <20% cost of energy by 2007 and <10% by 2012. None of the established and tested methods ( http://www.climatetechnology.gov/library/2003/currentactivities/sequester.htm; http://www.fe.doe.gov/programs/sequestration/) could demonstrate performance even close to these targets. The principal reason for failure is that most of these methods require a thermal swing to desorb the captured CO₂. This energy requirement is substantial. Consequently the US DOE and comparable organizations in Europe, Australia and Japan are investing in new methods searching for those that have promise of achieving these targets.

I have long been interested in CO₂ handling. When I started work as a scientist my training was in neurobiology, thus I became interested in CO₂ handling in neural tissues, which are extremely sensitive to changes in acidity, and in the CAs, the family of enzymes that rapidly reversibly converts CO₂ to and from the salt HCO3- (bicarbonate). CO₂ is the main waste product of the human body, and the body handles it by shuffling it into waste gases that are breathed out 20 times every minute, by the liver fixing it into urea, a process that removes both gaseous ammonia and CO₂ in the urine, a discovery reported by Nobel Laureate Sir Hans Krebs in 1932. The discovery of carbonic anhydrase (CA) in red blood cells was reported in 1932 by teams at Cambridge University and at the University of Pennsylvania. In the past decade, my interested in CA shifted from biological systems to applications of CA-based CO₂ capture devices for engineering purposes, e.g., by contained vessels such as space ships, and by the global environment.

One of these projects, now in DOE Phase III study, is under development by Carbozyme, Inc. (www.carbozyme.us). This approach uses the enzyme CA to facilitate CO₂ capture by converting it to bicarbonate, to allow ready deposition into a liquid film. This overcomes the substantial energy barrier to CO₂ absorption. Unlike, the most commonly used approach (for high pressure applications – amines) the energy required to desorb the bicarbonate from a CA-facilitated system is quite modest. CA is the most efficient catalyst known for CO₂. The particular isozyme used by Carbozyme has a maximal turnover of more than 1e6, i.e., it takes less than 1 µsec to capture one molecule of CO₂. All living organisms use CA to manage CO₂. The work in my Company has exploited the CO₂ handling system in the body by looking at the properties of CAs and a synthetic CA that is the main component of what is essentially an industrial scrubber, a prototype of this permeator is shown in Figure 2. Figure 3 is a schematic illustration of a massively parallel array of permeators as would be used in a commercial application.

In the Carbozyme design the CA is organized into a thin liquid film (~250µm thick) held in place between two microporous, hydrophobic, polymer films that in turn contact the feed gas on one side and the sw eep gas on the other. The liquid film, known as a contained liquid membrane (CLM) is a barrier to other gases in the feed dramatically decreasing the passage of nitrogen or oxygen, for example, while the CA at the gas-liquid interface promotes the uptake of CO₂ by immediate conversion to bicarbonate at the feed side. The bicarbonate diffuses across the CLM and is dehydrated and desorbed at the sweep side by a second coat of CA to exit in the sweep stream as highly humidified CO₂. This is a biomimetic process akin to tissues generating CO₂, the blood carrying the CO₂ as bicarbonate and the release of CO₂ at the lung to yield humidified, CO₂ enriched expired gas. This permeate gas is then compressed, dropping out the water (which is recycled) and dry CO₂ is forwarded to the pipeline. Its concentration is in excess of 95%.

Laboratory data and related numerical modeling have demonstrated each of the process steps. This includes the demonstrated ability to capture CO₂ from natural gas burners. Carbozyme is now gearing up for field test trials from coal burners. It will first devise methods to handle these more complex flue gas streams. Next it will build a fully instrumented permeator to demonstrate the control systems that are needed. Finally, it will do a long-duration demonstration (6 mos.) at 1 MW capacity. All of these tests will take about 1.5-2 years. They will develop the basis for a massively parallel array of membrane-CLM permeators needed to capture the 6,000 metric tonnes of CO₂ emitted daily from a 400 MW coal burning power plant burner. The process engineering carried out by Carbozyme indicates that the energy cost will be less than 13% while capturing 90% of the CO₂ to a concentration of >95%.

Figure 2. A Carbozyme CLM permeator for CO2 capture

The gray tube in the center is for temperature control. The CO2-rich FEED exits as the CO2-lean RETENTATE while the CO2-lean SWEEP is enriched to leave as the CO2-rich PERMEATE. The design shown is a dual hollow fiber, counter-current design with the feed and sweep gases on the bore side of the membrane and the CLM on the shell side. Each of these portals is labeled. The two ports allowing access to the CLM mean that it can be augmented, removed and replaced or otherwise modified without removing the permeator from its mounting rack

Figure 3. A schematic illustration of a massively parallel permeator array for CO2 capture

Following temperature control, the inlet distributes via a plenum to the permeators which strip out the CO2. The CO2-lean gas is sent to stack and the CO2-rich stream to the compressor that drops out the water, which is recycled, while the supercritical CO2 is sent to the pipeline. COTS = commercial-off-the-shelf equipment

Dr. Trachtenberg is the inventor, founder, president and CEO of Carbozyme. He and his group have competed successfully for over $3M in US federal grants related to this work. Dr. Trachtenberg grew up in New York City where he attended Stuyvesant HS, and The City College of New York (BA) before getting a PhD in anatomy and neurobiology from UCLA. He has been on the faculty of major medical schools in the U.S., a visiting scholar in Switzerland, and was formerly a co-founder and VP for R&D of NeuroGenesis a neutraceutical company that he helped to take public. He holds two patents with several pending. He has published over 100 articles. Contact Us for subscription Information Print Send email NewsLetter Tools Sponsored by