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 [1]. 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 [10]. 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 [4]. 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
[9]. 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 [4].
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 [11]. 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:
References