The objectives of this division are a) to develop highly efficient materials for CO2 separation in power generation and industrial processes; and b) to create energy efficient processes to convert CO2 into value-added chemicals such as liquid fuels or their intermediates.
In the area of CO2 separation, the goal is to develop novel membrane technology to separate CO2 from other gasses in the processes of precombustion for Integrated Coal Gasification Combined Cycle (IGCC), post-combustion at power plants, and at natural gas wells. While the CO2 selectivity of currently available membrane technology is sufficiently high for practical application, these membranes are still plagued by low gas permeability. In other words, insufficient throughput for practical application. One approach to overcoming this challenge is thinning the membranes, which are currently on the order of a few microns thick. Thus, the material design and development of thinner membranes for selective gas separation are central research topics in the division. CO2 conversion processes help reduce CO2 emissions because they can utilize or store otherwise wasted energy from intermittent renewable sources. The division seeks to identify and optimize suitable catalysts, electrodes, and associated operation conditions that allow for energy efficient, and ideally selective, electrolysis of CO2 into value-added chemicals such as CO, methane, methanol, ethanol, and/or ethylene. Current emphasis is on lowering or eliminating the precious metal loading (typically Ag, Au) in these cathode catalysts, switching to a Cu-based catalyst for the production of multicarbon products (ethylene, ethanol), identifying the best electrolytes (pH, conductivity) for each catalyst, and optimization of the gas diffusion electrodes (durability, quality of catalyst layer, porosity for fast reactant and product transport).
● DIVISION ROADMAP
● RESEARCH HIGHLIGHTS
In FY 2012, the Division revisited its research areas and in consultation with the Energy Analysis Division (EAD) refocused its efforts on capture, separation, and utilization technologies that hold promise for the future. The relevant scientific and technical challenges, which are outlined in the Division’s research roadmap shown below, are summarized as follows:
CO2 Capture and Separation
The CO2 separation cost is a dominant feature of the whole process of carbon capture and storage (CCS). Among conventional CO2 capture technologies, the solvent absorption method has been investigated and has gained current acceptance as the most accessible CO2 capture technology. However, it requires heating or additional energy for CO2 recovery after the absorption, which results in increased costs of CO2 capture. Table 1 summarizes the present and targeted costs of each of the current CO2 capture technologies.
Table 1. Separation technologies
* From METI’s report 2010
** From National Petroleum Council Report 2012
Energy is calculated from 1 kWh = 25 JPY (1 USD = 90 JPY).
According to this table, the solvent absorption method is not the most efficient process and alternative and game changing separation technologies are needed. Membrane separation and adsorption are promising candidates with great potential for efficient CO2 capture processes, at reduced cost. In order to fully understand and utilize the advantages of membrane separation and adsorption, we decided to aim at the separation and adsorption of pressurized CO2 gas emitted in pre-combustion and liquid natural gas extraction, as these sources emit a pressurized gas stream.
By considering and surveying of the current state-of-the-art in membrane separation of CO2, our priority target of source gas is a pressured mixture gas of CO2 and hydrogen, since hydrogen is valuable as an energy carrier. This mixture gas is generally emitted from an Integrated Coal Gasification Combined Cycle (IGCC) plant. In addition to the industrial aspects, such membrane separation presents serious scientific challenges. Because the CO2 molecule is larger than hydrogen, conventional size-based separation and capture do not work well. Furthermore, there is a contradictory problem in membrane separation, i.e. effective gas separation leads to smaller gas permeance and vice versa. In order to tackle this problem, the Division’s approach is based on membrane material design and ultimate thinning of the membrane, an approach that may also turn out to be useful for other gas separation technologies. Capture is also an issue. As part of the CO2 capture process, we will make efforts to develop porous materials, including MOFs, zeolites, etc.
Underground carbon sequestration is likely to be part of the strategy utilized to curb the increase in atmospheric CO2 levels. Sequestration is part of the now widely accepted “stabilization wedges” approach, introduced in a famous publication by Pacala and Soclow in Science in 2004 (Vol. 305, pp. 968-972). Many of the stabilization wedges, for example the capture and underground sequestration of CO2, come with a substantial net cost, and many have absolutely no potential for economic gain. In contrast, utilization of CO2 has also attracted much attention recently, since CO2 is one of the carbon sources to be converted to valuable compounds Thus, this project seeks to explore an approach (i.e., a potential additional stabilization wedge) that has the potential to earn back some of the cost of carbon capture through the electrochemical reduction of the captured and purified CO2 into useful chemicals. This process can be driven by the vast amounts of intermittent excess renewable power that is becoming available in many locations around the world. Furthermore, by utilizing CO2 as the starting material for chemical production, our dependency on fossil fuels is reduced. We strongly believe that the research in CO2 utilization is necessary to establish a whole scheme from CO2 capture to its utilization as an alternative option instead of CO2 storage. As mentioned above, CO2 capture, separation and underground sequestration is a highly costly process, with no economic return. Instead, CO2 capture, separation followed by chemical conversion (and no underground sequestration) has the potential to provide economic value!
● FUTURE DIRECTIONS
The Solvent absorption approach is the most established technology. However, implementation and commercialization requires further cost reduction. In I2CNER, the focus will be on CO2 separation with membranes and adsorbents which are expected to lead to more effective technologies.
CO2 Capture and Separation
Membranes (Taniguchi, Fujikawa)
The target of membrane separation is in an IGCC plant, where CO2 is separated over H2. A number of CO2 separation membranes have been developed, however, only a few successful examples can be found. The dendrimer membrane developed by Prof. Taniguchi et al. exhibits some of the highest CO2 separation performance. The separation factor (ratio of permeability) and CO2 permeance under pressurized condition are 30 and 1.0 x 10-10 m3(STP)/(m2 s Pa), respectively. However, for implementation, the CO2 permeance has to be raised to 7.5 x 10-10 m3(STP)/(m2 s Pa), whereas the separation factor is qualified. It has been found that CO2 permeance is inversely proportional to the membrane thickness (10-500µm), which indicates that CO2 permeation is diffusion controlled. Increase in CO2 permeance would be achieved by reducing the membrane thickness. Considering these aspects and the milestones, efforts will focus on material design and membrane thinning.
Porous materials (Kusakabe)
Exploration of various classes of absorbent materials is important given that organic/inorganic composite materials have great potential for the design of porous structures absorbing CO2. The purpose is to develop porous materials, including MOFs, and zeolites. In FY 2011, we have succeeded in preparing a large size MOF-5 which has porous crystal structure composed of organic ligands and metal ions. We will evaluate gas adsorption functions with this large MOF crystal and improve its CO2 adsorption and desorption properties by re-designing the structure based on the initial experimental results. In addition, preliminary results to make a membrane from this porous material on a filtration support have been obtained. This porous membrane on a filtration support also has great potential as an efficient CO2 separation membrane.
In FY 2012 Kenis, in close collaboration with collaborators from the fuel cell division, has identified three very promising catalyst materials for the selective conversion of CO2 to CO: (1) polymer-wrapped, multiwall nanotube supported Au nanoparticles – with Nakashima and Fujigaya as well as Gewirth; (2) an organometallic catalysts – with Gewirth; and (3) a metal free carbonitride catalyst – with Lyth. These catalysts start to approach the metrics needed for development of a commercial process: a Faradaic efficiency (FE) of >95%, an energetic efficiency (EE) of >60%, and a current density (CD) of at least 200 mA/cm2.
Considering these aspects and the research targets of the Division, our efforts in 2013 to 2014 will focus on three points: further characterization and optimization of these catalysts, and optimizing electrode structure, including implementation of these catalysts into these electrodes; and evaluation of the envisioned process with respect to its potential for commercialization.