As the prime mover of carbon through the atmosphere, carbon dioxide (CO2), plays a vital role in enabling the cycle of carbon from the Earth’s crust (where it is found in elemental graphite and diamond, carbonates, and fossil fuels) to our oceans

A. Das • D. M. D’Alessandro (H)

School of Chemistry, The University of Sydney, Sydney, NSW, Australia e-mail: deanna. dalessandro@ sydney. edu. au

A. Das

e-mail: anita. das@sydney. edu. au V. K. Peterson (H)

Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, Australia e-mail: vanessa. peterson@ansto. gov. au

© Springer International Publishing Switzerland 2015 33

G. J. Kearley and V. K. Peterson (eds.), Neutron Applications in Materials for Energy, Neutron Scattering Applications and Techniques,

DOI 10.1007/978-3-319-06656-1_3

(where it occurs in carbonate minerals formed by the action of coral-reef organisms and aqueous CO2). For hundreds of millions of years, the carbon cycle has main­tained a relatively constant amount of CO2 in the Earth’s atmosphere (approxi­mately 400 ppm by volume). While the contribution from human industry is relatively small, its recent growth has shifted this natural balance. Since the start of the Industrial Revolution around 1760, the concentration of CO2 in the atmosphere has risen dramatically from 280 to 385 ppm today [1, 2]. This significant rise has been attributed to an increasing dependence on the combustion of fossil fuels (coal, petroleum, and natural gas), which account for 86 % of man-made greenhouse-gas emissions, the remainder arising from land use change (primarily deforestation) and chemical processing.

The development of more efficient processes for CO2 capture from major point sources such as power plants and natural-gas wells is considered a key to the reduction of greenhouse-gas emissions implicated in global warming. Numerous national and international governments and industries have established collabora­tive initiatives such as the Intergovernmental Panel on Climate Change [3] (IPCC), the United Nations Framework Convention on Climate Change [4], and the Global Climate Change Initiative [5] to achieve this goal. The capture and sequestration of CO2, the predominant greenhouse gas, is a central strategy in these programmes as it offers the opportunity to meet increasing demands for fossil-fuel energy in the short to medium term, whilst reducing the associated greenhouse-gas emissions in line with global targets. Carbon capture and storage (CCS) will complement other strategies such as improving energy efficiency, switching to less carbon-intensive fuels, and the phasing in of renewable-energy technologies.

Three major technologies are predicted to have the greatest likelihood of reducing man-made emissions to the atmosphere that are implicated in global warming. These processes include postcombustion and precombustion capture from power plants involving CO2/N2/H2O and CO2/H2 separations, respectively, and natural-gas sweetening (CO2/CH4/N2 separation). The separation processes required for each of these capture applications differs with regard to the nature of the gas mixture and the temperatures and pressures involved, imposing constraints on the materials and processes employed [6, 7].

Conventional CO2 capture processes employed in power plants world-wide are typically postcombustion ‘wet scrubbing’ methods, involving the absorption of CO2 by amine-containing solvents such as methanolamines [8]. Power plant flue-gas streams consist primarily of N2, H2O, and CO2 in a 13:2:2 ratio by weight [9]. Prior to the compression and liquefication of the captured CO2 for transportation to storage sites, CCS requires the separation of CO2 from all other flue-gas compo­nents. CO2 is strongly absorbed by the amine to form a carbamate species [10], however, the high heat of formation associated with the creation of the carbamate leads to a considerable energy penalty for regeneration of the solvent. Since the flue streams from coal-fired power plants contain dilute concentrations of CO2 (typically 10-15 %) at relatively low pressures and temperatures (1 atm., 40 °C), it is estimated that CO2 capture and compression will increase the energy requirements of a plant by 25-40 %. Analysis has shown that the thermodynamic minimum energy-penalty for capturing 90 % of the CO2 from the flue gas of a typical coal-fired power plant is approximately 3.5 % (assuming a flue gas containing 12-15 % CO2 at 40 °C) [11]. The transportation and storage of CO2 will necessitate further investment and capital costs. These economic and energy comparisons underscore the immense opportu­nities and incentives that exist for improved CO2 capture processes and materials. Despite improvements in conventional postcombustion chemical-absorption meth­ods, wet-scrubbing methods suffer a number of drawbacks and are therefore not cost — effective for large-scale carbon emissions reduction.

While the retrofitting of existing power plants using postcombustion capture methods presents the closest marketable technology, two major alternatives to postcombustion CO2 capture processes have been proposed, and are currently in the test stages of development [12]. Precombustion processes involve a preliminary fuel-conversion step using a gasification process and subsequent shift-reaction to form a mixture of CO2 and H2 prior to combustion. The high pressure of the product gas-stream facilitates the removal of CO2 from the CO2/H2 mixture at pressures of 30-50 bar and temperatures of 50-75 °C [13]. The significant advantage of precombustion capture is that the higher component concentrations and elevated pressures reduce the energy capture penalty of the process to 10-16 %, roughly half that for postcombustion CO2 capture. A further advantage is that precombustion technology generates an H2-rich fuel, which can be used as a chemical feedstock in a fuel cell for power generation or in the development of an H2 economy. In oxyfuel (or denitrogenation) processes, fuel is combusted in O2 instead of air by the exclusion of N2, thereby producing a concentrated stream of CO2 without the need for separation (in high, sequestration-ready concentrations of 80-98 %). Since the separation of interest in this case is air separation (O2 from mainly N2 at a pressure of around 100 bar and temperature of 50 °C), reducing the cost of O2 generation is key to industrial viability. While the emerging technologies associated with precombustion and oxyfuel processes cannot be readily incorpo­rated (via retrofitting) into existing power plants as can postcombustion CO2 cap­ture processes, the projections from the IPCC indicate that the required extensive capital investments will be compensated by the relatively higher efficiency of the CO2 separation and capture process [3].

Another important application for CO2 capture technologies is the ‘sweetening’ of sour natural-gas wells, where the sweetening refers to the separation of CO2 from CH4. Natural gas reserves (mainly CH4) are typically contaminated with over 40 % CO2 and N2 and the use of such fields is only acceptable if the additional CO2 is separated and sequestered at the source of production. The capture of CO2 from ambient air has also been suggested, however, the low concentration of CO2 in air (0.04 %) presents a significantly higher barrier to capture compared with post­combustion methods, and the expense of moving large volumes of air through an absorbing material presents a further challenge in its implementation [6].

The key factor that underscores significant advancements in CCS is materials to perform the capture process [6, 7]. The challenge for gas-separation materials is that the differences in properties between the gases that have to be separated are rela­tively small. However, differences do exist in the electronic properties of the gases:

CO2 has a large quadrupole moment (13.4 x 10-40 cm2 vs. 4.7 x 10-40 cm2 for N2 and CH4 is non-polar) and CH4 adsorbs preferentially over N2 due to its higher polarizability (17.6 x 10-25 cm3 for N2 and 26.0 x 10-25 cm3 for CH4).

A diverse range of promising methods and materials for CO2 capture applica­tions that could be employed in any one of the abovementioned postcombustion, precombustion, or oxyfuel processes have been proposed as alternatives to con­ventional chemical absorption. These include the use of physical absorbents, membranes, cryogenic distillation, hydrate formation, chemical-looping combus­tion using metal oxides, and adsorption on solids using pressure and/or temperature swing adsorption, where the adsorption and desorption temperature/pressures are different and a “swing” is made between them [14]. The key requirements for these new materials are that they exhibit air and water stability, corrosion resistance, high thermal-stability, high selectivity and adsorption capacity for CO2, as well as adequate robustness and mechanical strength to withstand repeated exposure to high-pressure gas streams. A number of review articles have elaborated the status of a new classes of materials for CO2 capture [6, 7]. In particular, metal-organic framework (MOF) materials are progressing at a rapid pace.

With respect to new materials, the key scientific challenges are the development of a level of molecular control and modern experimental and computational methods. For crystalline materials, adsorption isotherms and breakthrough-curve measurements under conditions that closely resemble working-condition gas mix­tures are essential. In reality, pure gas-adsorption isotherms are often measured, with ideal adsorbed solution theory (IAST) applied in some cases to predict mul­ticomponent adsorption behaviour [15]. A parameter that must be assessed in all cases is the enthalpy of adsorption, since the cost for the regeneration of a capture material is clearly dependent on the energy required to remove the captured CO2.

Characterization of the molecular transport properties of materials is essential to obtain an understanding of transport processes. Important molecular-level infor­mation required includes: how the material structure changes with loading, how adsorbates bind to the material, and how different permeates influence each other’s solubility. In situ techniques are particularly powerful as they allow the interactions between gas molecules and the matrix to be probed and determine the material structure under different loading conditions. The most significant information from such measurements is gained from the ability to correlate absorbate uptake with the absorbent structure and the molecular-level absorbate mobility. A comparison between the molecular-level absorbate mobility and its macroscopic diffusion should provide insights into the mechanism of selective transport through these materials.

In parallel with experimental studies, computational modelling methods are being developed, both as a tool to understand further details of the adorbate — absorbent interaction, and as a tool to predict the performance of materials proposed for a given separation process, with the latter enabling large-scale screening of new materials. Ultimately, a clear understanding of the structure — and dynamics-function relations will direct experimental efforts towards a new generation of materials with improved CO2 capture abilities. Developing force fields for computational work using detailed structures is important for the successful prediction of thermody­namic and transport properties of new materials.