Carbon Dioxide (CO2) Capture, Conversion and Utilization-
Economical Sequestration of Carbon Dioxide from Power Plants
The project team is developing a highly selective gas separation membrane to capture carbon dioxide from the flue gas of a power plant. Polymeric based membranes for capturing carbon dioxide at room temperature or in a high temperature environment are being pursued. The team is also working on a new approach for the efficient conversion of CO2 into fuel (formic acid & methanol).
Goals of the research include:
1) Establish scientific basis for dry gas CO2 separation, adsorption, storage and conversion into fuel.
2) Establish proof of concept: efficient CO2 capture from flue gas and conversion for use in a hydrogen fuel cell in a near zero-energy loss power cycle.
The alternatives for CO2 sequestration such as geologic sequestration, conversion into other materials or deep ocean storage pose a safety hazard or are not economically sustainable. The background behind CO2 sequestration supports the research path that we are pursuing.
The greenhouse effect is the trapping of solar energy by certain gases of which the most important include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and chlorofluorocarbons (CFCs). Unprecedented and increasing emissions of CO2 have led to substantive climatic changes which are now supported by measurable data. These include the rise in the global mean air temperature over land and sea, satellite observations of microwave emissions from the atmosphere since 1979, records of the width and density of tree rings, changes in the extent of alpine glaciers, sea ice, seasonal snow cover, length of the growing season and increasing oceanic pH concentration levels . The current atmospheric CO2 concentration level of 364 ppm marks a 30% increase from the pre-industrial level of 280 ppm, which cyclically fluctuated by 3.5%. Figure 1 & 2 demonstrates the correlation between CO2 concentration level and the average temperature change. The evidence was further corroborated when the Intergovernmental Panel on Climate Change declared in 1996 that, “the balance of the evidence suggests a discernable human influence on climate change” [1,5]. As seen upon a closer inspection of figure 2, average atmospheric temperatures underwent fairly cyclic swings in the past, but substantial, acyclical increases in atmospheric temperature began after the industrial age and have risen even more sharply over the last two decades.
Recognizing the potential impact of global warming on life on the planet, 141 nations have come together to form a pact to mitigate CO2 and other greenhouse gas emissions. The pact, called the Kyoto Treaty, went into effect on Feb. 16, 2005 and calls for a 2% reduction in atmospheric concentrations of greenhouse gasses below 1990 levels, by 2012 . In order to meet this goal, treaty signatories will rely on the development of carbon sinks, new clean energy technologies and advanced technologies to curtail greenhouse gas emissions, and CO2 in particular, since it has the highest concentration in the atmosphere. Domestically, another green house gas mitigation approach has been adopted under the auspices, U.S. Global Climate Change Initiative which calls for a voluntary reduction emissions intensity (ratio of CO2 emissions to GDP) by 18% before 2012.
Despite the great body of research that has been pursued to control global warming through the sequestration of CO2 from power plants (the largest single source emitters of CO2), major shortcomings, namely environmental and safety concerns, inefficient capture rates and high costs still exist [2,3,5,7]. Though geologic sequestration is most popular for its attractiveness as a long-term solution, it poses grave human health and environmental threats . Any geologic shift could trigger the release of the CO2, which asphyxiates any life-form that relies on oxygen. This tragic reality was seen when over 1,700 people and thousands of animals lost their lives in Cameroon, West Africa in 1986 when the underground volcano beneath Lake Nyos unexpectedly released trapped CO2 . Most other alternatives for CO2 sequestration are prohibitively expensive. Deep ocean sequestration, in the form of hydrates, costs approximately $510/ton CO2, conversion of CO2 by biological systems and conversion to mineral carbonates vary from $80 to $100/ton CO2 .
None of the proposed technologies or research paths consider CO2 as a useful fuel source capable of lowering the energy requirement of capture. It is well established that CO2 can be converted into carbon monoxide (CO) with a 95% yield by heating at 800oC or through a low-cost catalytic process at 400oC. Several pathways exist for economically converting the CO to useful fuel: 1) A nickel catalyst can be used to convert the CO to methane (CH4) at 200-300oC in normal pressure; 2) A zinc oxide catalyst activated with copper and aluminum oxide can be used for conversion to methanol (CH3OH) between 200-400oC at 200 atm pressure; or 3) CO can be reacted catalytically with water to form hydrogen plus more CO2. Given the nominal waste heat temperature of 200-400oC available at the typical power plant, the conversion of CO2 could be done with no additional energy requirement. The energy produced from the fuel cell using any one of these three fuels could then be used to supply additional electricity to the power plant, thereby creating a near zero-loss power cycle. Refer to Figure 3 for a diagram of the project goal.
The key concerns that highlight the technological advantages of the UIUC CO2 sequestration project:
Important background information which explains the drive behind this project is explained here.
The underlying goal behind the UIUC CO2 Sequestration & Utilization Project is to capture CO2 in an efficient manner from the effluent of a power plant and convert it to a useful fuel source using the power plant's own waste heat. Previous work in Dr. Economy's research group has demonstrated the synthesis of polymeric membranes with high selectivity for acidic gasses . Recently, a low cost technique incorporating the polyethyleneimine (PEIM) polymer in a network of primary, secondary and tertiary amines coated on a glass substrate with crosslinked epoxy has demonstrated far superior selectivity and storage capacity for CO2. The new membranes are highly porous, mechanically strong, thermally stable at high temperatures and regenerable up to 4 times using temperature swing absorption. Refer to figure 4 for an SEM, FTIR scan confirming the PEI/epoxy structure and the regeneration profile of the membranes. The key properties that we are targeting include high selectivity, high reactivity, high adsorption capacity, low cost, good mechanical strength, stable adsorption capacity after cycles.
Figure 4: SEM, FTIR & Regeneration
The key areas of focus for the membranes include:
Efficient conversion of carbon dioxide into useful materials continues to be a lingering energetic challenge. Researchers have attempted to convert carbon dioxide using a number of techniques which include but are not limited to:
1. Thermally induced conversion to mineral carbonates
2. Conversion to O2 using algae ponds
3. Catalytic conversion to carbon monoxide (CO), methanol (CH3OH) and hydrogen (H2) using the following pathways:
Ø CO2 + C = CO * (800oC for 94% Conversion – FeC catalyst to reduce temperature)
Ø CO2 + CH4 = 2CO + 2H20 (300-400oC at norm press. – Fe catalyst)
Ø CO + H20 = H2 + CO2
Ø 2CO + 3H2 = CH4 + H2O * (200-300oC at norm press. – Ni or Co catalyst)
Ø CO + 2H2 = CH3OH * (200-400oC at 200-300atm – ZnO activated with Cu and Al2O3)
Our unique approach employs microelectrochemical cells, which utilize the waste heat of the power plant to reduce the energy of conversion. The captured carbon dioxide from the waste stream is first converted to formic acid which could be further converted to methanol for utilization in a fuel cell or storage and retail in the commercial markets.
Electrochemical conversion of CO2 to formic acid (HCOOH)
Electrochemical conversion of CO2 to methanol (CH3OH)
Figure 5: Electrochemical Cell for carbon dioxide conversion.
Key opportunities envisioned from the project include: