Faculty Affiliate, ISSP
Full Professor, Department of Chemical and Biological Enginnering, Faculty of Enginnering, uOttawa
On Thursday, May 27, the ISSP hosted Food for Thought: Application of Adsorption Processes for Carbon Capture and Thermal Energy Storage. This blog is an adaptation of the speaker’s remarks.
Carbon Capture and Thermal Energy Storage technologies using “Adsorption Processes” have very important roles to play to mitigate the effects of climate change. In this blog, I am going to summarize how we can use these technologies and how they would help.
To start with, let us talk about what Adsorption is: Adsorption should not be confused with Absorption. Both of these processes are commonly used for gas separation applications. In the case of Adsorption, the material that does the sorption is a highly porous solid and the sorption happens on the surface. In the case of Absorption, the material is a liquid and the whole volume of the liquid does the sorption.
Adsorbents have following characteristics:
- They are solids with high porosity (similar to a sponge);
- Some have extremely high surface area (larger than 500 m2/g);
- Some have micro-pores of few Å size (1 Å is equivalent to 10-10 m);
- Some have molecular sieving properties, since the gases we want to separate has dimensions in the order of Å.
Some of these adsorbents have unique pore sizes, like the sieves the children play with in the sandbox; they can sieve and separate the molecules from each other. In the case of the sieves in the sandbox, we hold the sand pieces larger than the size of the holes of the sieve. In the case of the molecular sieving, we hold the molecules that are small enough to go into the small pores of the adsorbent. On top of the size selection for these adsorbents, there are some interactions between the adsorbents and the host molecules that effect the sorption. These adsorbents will eventually be saturated and they will need to be regenerated (during desorption) to remove the gas molecules trapped in their system so that they can be used again to trap the molecules. Therefore, all adsorption processes work in cycles of adsorption, followed by desorption, which will form one complete cycle.
These cycles are repeated on an on-going basis with columns filled with the porous adsorbent material. In order to have a continuous flow of inlet feed gas, at least two columns are needed, so that while adsorption is happening in column 1 while the feed is entering this column, desorption will be happening in column 2 where the trapped molecules are recovered and the adsorbent in column 2 is ready to adsorb again. Once column 1 gets saturated, then the flow is switched in such a way that the feed would be entering into column 2 for adsorption to take place in this column and desorption will take place in column 1, where the trapped molecules will be recovered from that column. These cycles will continue and you would have a continuous feed flow into the overall system, as well as a continuous output flow from the overall system.
In the case of carbon capture technologies, carbon dioxide gas is captured either from combustion gases (such as power generating plants, or anywhere where fuel combustion happens) or directly from ambient air using this cyclic adsorption process. It is then concentrated during the regeneration for carbon dioxide to be either stored underground or used as a raw material to make synthetic fuels or other value added chemicals. This will prevent the carbon dioxide gas to be released into the atmosphere in the case of combustion gases, and it will decrease the concentration of carbon dioxide in the ambient air in the case of it being captured from air. Since carbon dioxide is a greenhouse gas, its concentration in the air needs to be decreased if we want to reverse the negative effects of climate change. We have to innovate technologies and processes for carbon dioxide not to be released into the air during any process. We also need to reduce its concentration in the air. Adsorption processes would play an important role in helping with these objectives. At University of Ottawa, with financial contributions from NSERC (Natural Sciences and Engineering Research Council) and Natural Resources Canada, we are working on developing new adsorbent materials as well as new processes to achieve these objectives.
Adsorption technology can also be used for “Thermal Energy Storage” by using the exothermic characteristic of the adsorption process. All exothermic processes release heat as they take place. If we pass moist air through a column packed with a highly porous adsorbent, moisture in the air will be adsorbed in the pores of the adsorbent and this exothermic process would release heat that would be taken up by the air carrying the moisture in the column. This heated air can be used for space heating applications. This adsorption process will be the “Discharging of the Thermal Battery”. Once the column gets saturated with the moisture, it can be regenerated (water can be desorbed from the column) by using heat that could be provided from a solar panel or by using any low-quality waste heat (such as heat provided by a stream at a temperature lower than 200oC). This desorption process would be the “Charging of the Thermal Battery”. The adsorbent materials would be able to handle numerous multiple cycles of these charging and discharging applications.
The advantages of Adsorption Thermal Batteries are as follows:
- There are no toxic compounds or chemicals. Only the air that we breathe, the water that we drink ,and the adsorbent materials, which are mostly made out of materials that have properties similar to earth that we walk on;
- Thermal energy can be stored for a long time so that weekly, seasonal and yearly storage applications can be handled without losing the charge of the thermal battery;
- They can be built at any scale, small or large, depending on the application;
- They are portable.
At University of Ottawa, we are developing materials for increased energy storage densities, as well as developing processes to increase the efficiency of the process.