Whereas the previous processes focused on the use of CO2 to form calcium carbonates, thereby storing the emissions in building materials, the processes that are used by companies such as Skyonic7 and Mantra8 aim to convert the CO2 into other chemicals, such as baking soda, hydrochloric acid, bleach and formates, which can be used in chemical manufacture and fuel cells. The Skyonics project is in the commissioning phase at a cement plant in Texas. Mantra is working on the design of two pilot projects, one of which will be at a cement plant in Canada.
Algae and microbes
Many years of research has gone into understanding and optimising the use of algae as a natural sink for CO2, mostly in the realm of biofuels. Although some hurdles still need to be surmounted, progress has been (and, most likely, will continue to be) substantial. Let’s take a look at what the requirements are for algae to grow, before discussing their applications.
Algae are incredibly easy and quick to grow, given the right conditions. All that they require is: sunlight, water, carbon dioxide and a few inorganic nutrients. With these, and the right atmospheric conditions (such as temperature and acidity), algae can produce more biomass than terrestrial plants per unit area, and unlike terrestrial plants, they can be grown on otherwise infertile land by using either saltwater or fresh water. Typically, one (dry) t of algal biomass will consume approximately 1.8 t of CO2 during its growth cycle (depending on species), and so they make excellent carbon sinks.9 There are two main forms in which algae are currently being cultivated: open ponds and photobioreactors.
Open pond systems are already commonly used in the United States. They typically use shallow (approximately 1 ft deep) ponds, ranging in size from one to several acres, and commonly have some form of water moving device installed to keep the algae circulating. They are often called ‘raceway’ ponds, due to their similarity in shape to a race track. With the aforementioned CO2 and nutrients, enough algae can be grown to be harvested within approximately 10 days, depending on the conditions and genus of algae used.
Harvesting the algae typically involves a two-step process (but there is some room for variation depending on the particulars of the project). In the first step, a portion of the water is harvested before concentrating the algae for removal. At that point, the biomass can be further processed (if, for example, the point is to produce some form of biofuel), or it can be dried (to be used for animal feed or other nutritional products).
Although open pond systems are substantially cheaper to construct and maintain, there are some drawbacks. First, due to their nature, it is very difficult to prevent contamination of the pond by invasive species of algae and bacteria. Furthermore, systems using a monoculture (as opposed to a diversity of strains) can be susceptible to viral infections.
Photobioreactors (PBR) take the acres of land used in open pool systems, and instead condense their footprint by running nutrient-rich water through clear plastic tubes, typically in parallel. The water is then exposed to sunlight (real or artificial); hence the name. While significantly more costly, and perhaps more difficult to use than open pond systems, PBRs offer a higher level of control over the operational parameters required to grow the biomass. Furthermore, because of their condensed footprint, PBRs can be used in colder climates (potentially year-round) that would normally be unsuitable for open pond systems, by setting up the PBRs in either a greenhouse or other enclosed structure. In addition, artificial lighting can be used in conjunction with PBRs to allow algae growth over night. Microbial strains offer a further advantage over algae as they can be grown in the dark, thus eliminating the need for light exposure.
As previously mentioned, the lion’s share of interest in algae is in its ability to convert the captured CO2 into some form of biofuel. In 2007, the United States passed the Energy Independence and Security Act (EISA), which set major goals for the reduction of greenhouse gas emissions by developing biofuel production with a capacity of 36 billion gallons by 2022. By 2013, the National Alliance for Advanced Biofuels and Bio-products (NAABB) was able to reduce the cost of biocrude to roughly US$7.50 a gallon, mostly through innovations in identifying new algae strains, and cultivation and harvesting methods and technologies.10 Another factor that allowed the NAABB to achieve such a low cost was by looking at the feasibility of harvesting algae for other products, such as animal and mariculture feed and fertilizer. Other potential products include plastics, antioxidants, flavours, fragrances, colorants, proteins and fatty acids.
These higher value products may be of greater interest to the cement industry than biofuels, due to the economics. Not only would partnering with an algae growth firm benefit the cement company by removing their CO2 emissions, but being able to sell the product for a higher price than what is required to run the system may make the economics of retrofitting an existing cement plant with PBRs more enticing. As it stands, there are presently several projects that are underway as partnerships between cement companies and algae firms.
Algae and microbe projects have the advantage of using the raw flue gases without the need for concentrating the CO2. Some algae and microbe strains can endure and even consume other pollutants in the gases, such as NOX and SOX, further improving the economics of these types of projects. It may be for these reasons that cement companies have shown such a great interest in this technology.
The elimination of CO2 emissions from cement manufacture is a daunting task and will require a full array of solutions. As mentioned at the beginning of this article, simply sequestering the captured CO2 in geological formations as a long-term solution may not be sustainable. By looking at using or converting the captured CO2 into useful products, cement plants can reduce or completely eliminate their CO2 emissions, while potentially receiving economic incentives/advantages. More important, however, is that the CO2 will essentially be removed from the atmosphere.
There are three main areas of apparent interest in the cement industry today. The use of CO2 in building materials is one area where Calera is making fine mineral carbonates that can be used as fillers or aggregates, Solidia is making a type of low carbon cement that is carbonate bonded, while CarbonCure is directly utilising waste CO2 in fresh concrete production to improve its characteristics. The direct conversion of CO2 from flue gas into useful chemicals is being pursued by Skyonics and Mantra, while many cement companies are exploring the viability of algae and microbes. It may take a combination of all these technologies and possibly many more to completely eliminate CO2 emissions from cement production; however, the projects mentioned here prove that innovation is ongoing.
9. ‘Accelerating the uptake of CCS: Industrial use of captured carbon dioxide’, Global CCS Institute.
10. NAABB Synopsis report. http://www.energy.gov/sites/prod/files/2014/07/f18/naabb_synopsis_report_0.pdf
This is the final part of a three-part article written by John Kilne and Charles Kline for World Cement’s August 2015 issue and abridged for the website. Subscribers can read the full issue by signing in, and can also catch up on-the-go via our new app for Apple and Android.
Read the article online at: https://www.worldcement.com/the-americas/31122015/carbon-utilisation-part-three-20/