Methods and Techniques for CO2 Capture

When comparing methods for capturing carbon with the least expense, businesses consider planting trees, mangroves, sea grasses, cover crops, etc. to offset the estimated volumes of CO₂ that they are releasing into the atmosphere. They estimate that for every ton of CO₂ emitted, one tree needs to be planted and reach maturity. However, the carbon captured from new land and aquatic planting needs large amounts of water and land to grow, and energy to harvest and transport.

Some companies capture CO₂ emissions during the process of generating electricity or heat. Others capture directly from the atmosphere for ‘direct air carbon capture and storage’ (DACS) using industrial-scale fans to draw air into the DACS system.  The captured CO₂ can then be compressed under high pressure and pumped via pipelines into deep geological formations such as depleted oil and gas reservoirs, deep saline formations, and un-mineable coal seams.  This permanent storage process is known as ‘sequestration’. Oil and gas companies have been pumping or injecting carbon directly into the ground before it reaches the atmosphere to enhance recovery of fossil fuels.

Alternatively, the CO₂ can be pumped under low pressure for immediate use in commercial processes, such as carbonating drinks or cement manufacturing. 

Some scientists are engineering microbes to “eat” carbon monoxide and carbon dioxide and produce chemicals like acetone and isopropanol, which are used in disinfectants and rubbing alcohols, and are the basis for acrylic glass and polypropylene plastics.

While of great significance, these methods cannot sufficiently sequester the billions of tons of CO₂ emitted every day that need to be either captured or reduced. 

We focus on methods that capture carbon from various sustainable sources called renewable feedstocks, which vary in cost, supply reliability, quality, and efficiency for downstream processing into products that can replace petroleum-based products such as biofuels and biobased durable goods that take much longer to break down back into carbon. Renewable feedstocks include agricultural wastes (herbaceous residues, woody residues, crop residues), municipal solid wastes (that contain an average of 40% organic material, food waste), gaseous resources (renewable natural gas, methane), wet resources, and plastics.

We first highlight microalgae for both carbon capture and as a much-overlooked renewable feedstock. Algae produces more oxygen than the world’s rainforests, absorbing 300x more CO₂ than a tree and 700x more than most land plants1. While the term ‘algae’ can refer to a large group of diverse, sometimes unrelated species of photosynthetic organisms that grow in fresh and salt water from unicellular microalgae to giant kelp, we assess ways to improve the cultivation of microalgae because it is easy to grow anywhere that temperatures are between 0 and 34 degrees C° as long as it has light, water (including saline, brackish and recycled water), and CO₂. 

Today, microalgae is being grown 24/7 in bioreactors - bags or tubes made of transparent materials (plastic, glass) - and located anywhere: indoors, including inside unused building space, basements, garages, and outdoors on marginal, inarable land, and even in deserts.  They can be installed on building and factory rooftops where they capture CO₂ from the air or fed directly from flue pipes. Some architects have clad the outside of buildings with bioreactors. Algae can also be grown in ponds – indoors (covered tanks) or outdoors in most temperate climates (also on inarable land).  Some algae ponds today are larger in size than seven football fields.

Since the costs to cultivate and transport dried algae are low when compared with other crop biomass, algae can be considered the most efficient crop for CO₂ capture on the planet. 

Algae can also be considered a prized renewable feedstock for harvesting, drying, and conversion into biofuels and biobased coproducts that can replace thousands of tons of petrochemicals and fossil fuels because it can:
• grow faster and with higher tensile strength than any other terrestrial crop
• be grown for its content of oils, carbohydrates, or proteins
• produce as much as 70% lipids based on dry cell weight
• utilize sugars and other carbon sources

Compare the lipid yields of algae for biodiesel with other seed and grains crops.
Since algae can be cultivated for its oils, carbohydrates, or proteins, it can be converted into a plethora of high-value downstream products:
• bioethanol, succinic acid, carboxylates and 2,3-BDO for biofuels
• HDO and HI for renewable diesel  
• PUFAs to polyurethanes, Sterols to surfactants
• Polyurethanes, plastic composites, algal growth media, HTL-based biocrude, carbohydrates, biogas, thermoplastic NIPUS; foams and resins; materials for packaging

Since fossil fuels had been comparatively inexpensive compared to biofuels over the past ten years, the algae biomass market pivoted to develop uses in higher-margin biobased co-products:
• animal feed
• food (meatless burgers, and other proteins that reduce the carbon footprint of industrial agriculture)
• beta-carotene and omega-3 fatty acids for nutraceuticals
• cosmetics, etc.
• Misc.: fertilizers, in laboratories (agar), as gelling agent in foods, in medicine, ink for 3-D printers, to remediate wastewater and to control pollution (mainly nitrogen and phosphorous), cement (an industry responsible for more than 8% of global carbon dioxide emissions – more than three times the emissions associated with aviation.

The total algae biomass market is projected to grow at a CAGR of 10.9 from 2022 to 2031 to reach $55.67 billion2. Today, manufacturers wishing to offset carbon emissions and “go green” are incorporating algae-based materials into their products – e.g. NIKE sneaker toppers, flip flops, and surf boards.

Aequor was invited to address several challenges facing algae cultivation at commercial scale as a member of the U.S. Department of Energy (DOE)’s DISCOVR Consortium. At one of the participating National Labs, Aequor developed a non-toxic, “green’ drop-in water treatment for use in any size algae cultivation system that solves process inefficiencies such as clumping (aggregation of biomass that blocks sunlight and nutrients from reaching the microorganisms), and equipment contamination. In bioreactors, Aequor’s treatment boosts algae biomass by 40% in half the time, eliminating the need for costly capital equipment to disrupt the aggregations, enzymes, biostimulants, engineered algae strains and biocides. In open ponds, Aequor’s treatment uniquely protects the algae crop from predator amoeba responsible for incessant pond “crashes.” A technoeconomic analysis undertaken by the DOE confirmed Aequor’s treatment could lead to significant savings in water, energy, labor, and downtime in open pond algae cultivation, and extend as harvest cycles by weeks, making large-scale algae cultivation profitable.

Although algae is also the renewable feedstock that is least likely to experience a shortage because it can be grown so quickly indoors, outdoors, etc., its production capacity pales compared to the abundant agricultural residues (grain, seed and land crop and forest waste) that are ground and fermented with yeast into bioethanol.  In fact, bioethanol is today the #1 U.S. agricultural export as countries blend it into gasoline at the pump as the fastest way to reduce carbon emissions.  The yeast biomass market for sustainable co-products was recorded at $30 billion in 2017 and projected to reach $130 billion in 2025, with biofuels accounting for $132 billion by 20233. Expect a boom in innovations and industrial-scale production of biofuels and coproducts in the U.S. due to provisions in the U.S. Inflation Reduction Act that support the Bioeconomy.

As experts in “green” remedies to control fouling and biocontamination, Aequor was invited to work with one of the U.S. National Labs under a Department of Energy program to address operational inefficiencies in the fermentation of cellulosic residues into bioethanol. It is widely known that bacteria rapidly attack the yeast used in the fermentation process. This causes biomass production inefficiencies and rapid failure, that requires shutting down and decontaminating the system, and reseeding it to start the process again. Aequor developed a drop-in formulation for any size fermentation system using common stains of algae and yeast that boosts profitability by up to 30% in less time and without the need for antibiotics or disinfectants, contributing to a safe, reliable – and profitable – supply of biomass at scale and contributing to the production of more stable biofuels.

Biofuels from cellulosic ethanol are assessed for their stability and storage life compared to fossil fuels. Water can eventually separate out of the biofuels, which rapidly contaminates with bacteria that form biofilm slime, corrosion, and other inefficiencies in storage tanks, pipelines, injectors, components, etc. Aequor has developed products to address stability issues to encourage widespread adoption of renewable fuels.

Since the transportation industry is considered the #1 emitter of greenhouse gases and widespread electrification will take years to be adopted, additional government incentives for the agricultural sector enable the commercial-scale development of greener fuels from renewable feedstocks for cars. Ethanol is considered the highest-octane additive commercially available, increasing mileage and reducing air pollution from fine particulate matter (PM) emitted in gasoline exhaust. Replacing fossil-based aromatics with bioethanol that is blended with gasoline at the pump is today considered the quickest way for countries to meet their UN carbon emission reduction targets. It is estimated that ethanol at the current 10% blend level in the U.S. is already displacing more than 8 billion gallons of aromatics; blends up to 30% are projected to be phased in shortly. Bioethanol from crop residues has been the fastest growing U.S. agricultural export for the past five years to help many countries meet their emissions reduction commitments.  
Other biofuels include biodiesel, renewable diesel, and sustainable aviation fuels (SAFs). Aviation accounts for 2-3% of global CO₂ emissions and air travel is expected to double in the next 15 years. The International Air Transport Association (IATA) has already committed to achieving carbon neutral growth from 2020 onwards and net-zero carbon emissions by 2050. The use of SAFS reduce greenhouse gas (GHG) emissions by up to 80%* compared to fossil jet fuel. The major airlines are signing on to use SAFs bolstering demand for renewable feedstock production.

Author: Marilyn J.  Bruno, Ph.D.
[email protected]

¹Algae-fueled bioreactor soaks up CO₂ 400x more effectively than trees. (2019) Retrieved from https://newatlas.com/environment/algae-fueled-bioreactor-carbon-sequestration/

²Algae Market. (2022) Retrieved from https://www.transparencymarketresearch.com/algae-market.html  

³Market Research Report. (2020) Retrieved from https://www.fortunebusinessinsights.com/cellulose-market-102062

Printed in Issue 2, 2022 of Carbon Capture Magazine