CO2: a valuable alternative carbon source
In recent years, carbon dioxide (CO2) has been steadily attracting attention as a valuable source of carbon. Of course, the rising concentration of CO2 in the atmosphere is a major and growing concern, and this is driving an increasing focus on sustainability in society. In line with this, we at Thomas More are working to achieve a more circular economy and a more bio-based economy. This means obtaining materials, chemicals and energy from renewable (energy) sources, and not from fossil fuels. Alternative biomass could become a major source in this approach.
Currently, the main activity of our research group is cultivating renewable biomass, partly in the form of algae. We’re doing this under controlled conditions in the horizontal tubes of a photo bioreactor. We use pure gaseous CO2 as the source of carbon. We are cultivating algae with a view towards various applications. Algae can be very useful in the cattle feed sector, for instance, or in the food sector, the health products or ‘neutraceuticals’ sector or the cosmetics sector. Our research group is not heavily involved in further developing these applications – we’re focusing on optimizing the cultivation of the algae, or in other words the process technology aspect.
Algae for conversion into valuable raw materials
Micro-algae form a really large and diverse group. More than 50,000 different species of algae have been identified and there are probably many more, running into hundreds of thousands. They are single-celled organisms, but can sometimes also form colonies. Algae are photoautotrophic organisms, which means they use CO2 as a source of carbon and then convert this into sugars by means of photosynthesis. The micro-algae that we cultivate contain a particularly large amount of interesting substances: proteins, sugars and fats being the main groups. In addition, the micro-algae also make high-value chemicals such as pigments and antioxidants. To give one example, we at radius cultivate a special alga that produces the valuable red colorant phycoerythin. You can pretty much regard algae as tiny factories that can produce all kinds of substances that we need – so in order to synthesize these substances, we don’t need to completely reinvent the wheel. The various algae cells have evolved under evolutionary pressure to make these interesting substances, simply using a little sunlight, CO2 and a few nutrients. That means there’s a huge potential for utilizing these substances.
An algae culture increases in density through cell division. If conditions are right, then the algae will continue their cell division until a culture reaches its maximum density. At this point, the algae are harvested, so the algae biomass itself is the product. In our closed photo bioreactors, we achieve a density of 1 to 2 grams of dry material per liter. When this point is reached, we take the algae out. This biomass can be directly used for food purposes or as cattle feed, but we can also further process the biomass, ‘break it open and extract the most interesting substances. If we take this latter approach, it’s called bio-refining or extraction. The whole process of cultivating, harvesting and further processing the algae presents a major challenge. That’s because each step is important and has to be carried out as efficiently as possible to ensure that the entire operation is profitable.
Mass flow controllers for precision flow of CO2
To optimize growth, it’s important to select an alga that grows well under the conditions we can provide in our unit. Not all algae species can absorb CO2 with the same efficiency, and not all algae grow equally fast. In our research, we find out which temperatures are best for growing the various species of algae, and how much light a particular alga needs. Here on the campus, we use natural sunlight: the photo bioreactors are in a greenhouse. As a result, the algae grow during the day, when the sun shines, and not at night. One of the research questions we are investigating as part of the ‘EnOp’ Interreg project is: if we add extra CO2 to the reactor, how much faster will the algae grow, and which algae types absorb the CO2 most efficiently? In order to answer this question, we need mass flow controllers, because we want to know exactly how much CO2 we have added.
The CO2 is mixed with in flowing air that is channeled to the reactor, after which the CO2 dissolves in the liquid culture fluid, which also contains other nutrients. Since CO2 (carbon dioxide) is a weak acid, the pH level of the fluid steadily falls. This has a negative effect, because most algae grow best at a pH level between roughly 7 and 8. However, as the algae grow, they absorb CO2 from the fluid, making the pH rise again. The acidity level is a highly critical factor – if the pH moves outside the desired zone, then the algae tend to flocculate. The dosing system is therefore linked to the pH level, to optimize the supply of CO2 as precisely as possible. In this way, we can establish the maximum growing speed of the alga and how much CO2 we need to add to achieve this.
If we add too much CO2, then the pH of the fluid will fall too strongly, and the algae won’t grow enough. If we don’t add enough CO2, that in itself isn’t a problem, but the algae will grow more slowly, because their growth is limited by lack of carbon dioxide. For each alga, an optimum amount of CO2 can be added. Moreover, the CO2 needs to be given time to dissolve in the fluid. If the CO2 doesn’t dissolve, then it will ultimately escape from the reactor again, which means you’re simply wasting CO2. Whether the CO2 is effectively dissolved and absorbed therefore needs to be taken into account as well. The design of the reactor plays an important role in managing this aspect.
As you might have noticed, precision is very important in this process. The mass flow controller ensures that we can keep the whole process stable around the right pH level and that we know exactly how much CO2 has been added.
…and the future?
If this process is scaled up to actual production scale, then logistics will become a major factor in determining where the CO2 comes from. In principle, it’s possible to use exhaust gases straight from factories, but then you need to remove substances like sulphur oxide and nitrogen oxide, which are also present in these flue gases. If the levels of these substances are too high, they will inhibit the growth of the algae. There are technical solutions to this problem, however. The next question is: how far away can the algae factory be from the CO2 source? If this distance is too great, then the CO2 will have to be transported in another, controlled form, such as bicarbonate. Another option is to develop CO2 air-capture units that enable local extra CO2 to be extracted from the air. The University of Twente is working on this technology in another Interreg algae growth project, known as IDEA and currently running in North West Europe. The Radius research group at Thomas More UAS is also involved in this project. In technological terms, we know it’s possible, but the crucial point is how much the technology will cost.