Author / Liang Jyh Chang, Adjunct Professor, Department of Chemical Engineering, National Taiwan University and Tsing Hua University 

In the first part of this series ^[1], it was mentioned that the measures to reduce carbon emission in the petrochemical industry can be summarized in an acronym SCUR which represents 4 words: Save, Capture, Utilization, and Renewable/Recycle. The first two articles in this series introduce Carbon Capture and Storage (CCS) and energy saving ^[1,2]. In this article the Utilization (CCU) part is discussed. The objective is to introduce a variety of ways carbon dioxide could be used to reduce carbon emission.

The role of CCU in the carbon reduction community is a bit controversial. On the one hand who doesn't like making use of a hated component and in the meantime also recoup some of the carbon capture costs. For researchers, improving conversion pathways for CO₂ is an interesting topic with many potential pathways to investigate. However, in my view CO₂ use is a complement, not an alternative, to CO₂ storage for large-scale emissions reductions. CO₂ use is not expected to deliver emissions reductions on the same scale as carbon capture and storage (CCS), but can play a role in meeting climate goals as part of an "all technologies" approach. Both Intergovernmental Panel on Climate Change (IPPC) and International Energy Agency (IEA) also hold a similar view ^{[3, 4]}.

There are some fundamental reasons why CO₂ utilization's role may be limited. They are Iisted below so that readers can keep them in mind as we go through the various potential utilizations:

1. CO₂ is a very stable molecule. To convert to a much more reactive CO currently requires an energy input of 380-490 KJ per mole.
2. Most of the CO₂ utilizations converts it to hydrocarbons which means hydrogen has to be added as a reactant. Hydrogen is a highly valuable molecule by itself. It can be used as an energy source for fuel cells or carbon-free combustion. It is also increasingly in demand to hydroprocess refinery products to reduce sulfur and upgrade heavy oils.
3. In addition, as of today, hydrogen production is a highly energy consuming process by itself. Hydrogen production by electrolysis of water needs about 50 Kwh of energy for each kg of hydrogen which is nearly 10 times that of steam methane reforming.
4. CO₂ reduction through utilization could be very short-term. Many of the products from CO₂ are then consumed and generate…. CO₂. So the reduction could be just a temporal effect. Proponents advocate that this could be a step toward CO₂ circular economy and reduces the use of fossil fuel.
5. The scale of CO₂ utilization needs to be very large to make a dent in CO₂ emission.

Given these reasons it is still important to keep an open mind about CCU's potential because incentives and regulations from government as well as new technology development could potentially change the economics.

Media in Taiwan often gets excited about the CO₂ utilization news. It is my hope that this article could help readers look at the news or relevant information in a rational way and assess the true impact of the CO₂ applications.

Many of the utilization consumes a lot of energy in the process. The companies that made news often brush off the negative effect of the carbon increase from energy consumption by claiming that the energy would come from renewables. If we do come to a point that renewables become so abundant and inexpensive then the economy of CO₂ utilization could shift. For now, the economic feasibility remains the biggest challenge.
In this article I will introduce the CO₂ applications that are currently in use today and those emerging ones that are technically (but may not be economically) feasible.

I. CURRENT STATUS

CO₂ can be a commercial input to a range of products and services. Today, around 230 million tons (Mt) of CO₂ are used each year ^[3]. The largest consumer is the fertilizer industry, where around 130 Mt CO₂ per year is used in urea manufacturing, followed by the oil sector, with a consumption of 70 to 80 Mt CO₂ for enhanced oil recovery (EOR) ^[3]. CO₂ is also widely used in food and beverage production, the fabrication of metal, cooling, fire suppression and in greenhouses to stimulate plant growth.

Figure 1 from ^[3] provides a breakdown of the CO₂ consumption in various industries and the projected growth rate. Geographically more than two-thirds of current global demand for CO₂ comes from North America (33%), the People's Republic of China ("China") (21%) and Europe (16%), with the demand for existing uses expected to grow steadily year-on-year (Figure 1).

 

Figure 1. Growth in global demand of CO₂ over the years and breakdown of demand in 2015. ^[3]

 

Although 230 Mt may seem a lot it is only 0.7% of annual CO₂ emission. This is the reason why unless there are major new utilizations that significantly increase the volume the impact of CO₂ utilization on carbon reduction would be limited.

Below is a brief introduction of the main existing utilizations today. The consumption of CO₂ in these applications are expected to grow along with global economy and population increase. However, we cannot expect a step jump to significantly mitigate CO₂ emission.

 

A. Urea for Fertilizer

Urea, also called carbamide, is an organic compound with chemical formula CO(NH₂)₂. It is a colorless, odorless solid, highly soluble in water, and practically non-toxic. The main reaction that produces urea is by combining ammonia and CO₂. 

2 NH₃+ CO₂ ⇌ CO(NH₂)₂ + H2O

More than 90% of world industrial production of urea is destined for use as a nitrogen-release fertilizer. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use. Therefore, it has a low transportation cost per unit of nitrogen nutrient.
Other usages of urea include resins, explosives, and automobile's NOx-reducing converters ^[6]

B. Enhanced Oil Recovery (EOR)

After discovery of an oilfield, it is initially developed and produced using primary recovery mechanisms in which natural reservoir energy (i.e. expansion of dissolved gases, change in rock volume, gravity, and aquifer influx) drive the hydrocarbon fluids from the reservoir to the wellbores as pressure declines with fluid (oil, water, or gas) production. This is "primary oil recovery" which ranges only between 5 and 20 percent ^[7] of the original oil-in-place (OOIP).

These low recoveries prompt field operators to find ways to improve recovery through the application of "secondary recovery" methods, which provide additional energy to the reservoir. Secondary recovery methods entail injecting either water and (or) natural gas into the reservoir for repressurizing the well and to potentially act as a water and (or) gas drive to displace oil. This helps to sustain higher production rates and extends the productive life of the reservoir. Normal practice has been to inject natural gas into the gas cap or at the top of reservoir and inject water below the oil-water contact.

The combined primary and secondary oil recovery is reported to be in the range of 20–40 percent of the OOIP ^[7]. As a result, there is a large volume of potentially recoverable oil left in the reservoir, which becomes the target for a suitable EOR processes. Of the various EOR processes, CO₂-EOR is the most widely used process with the highest potential for additional recovery.

Because of its special properties, CO₂ improves oil recovery by lowering interfacial tension, swelling the oil, reducing oil viscosity, and by mobilizing the lighter components of the oil ^[7]. Water is then injected to help push out the lower viscosity oil. The oil is piped to shore and CO₂ is re-injected to do more EOR and ultimately permanent storage.

A Youtube video by Scottish Carbon Capture and Storage (SCCS) provides a good visual introduction of EOR ^[8].

 

C. Food and Beverage

CO₂ has a wide range of applications in the food and beverage industry ^[9].

• The most obvious is the gas used for carbonating beverages, in particular beer, soft drinks, and wine. Besides adding to beverage enjoyment CO₂ also prevents the growth of bacteria and fungi. Sparkling water consumption is over 30% of all bottled waters in many European countries.
• Carbon dioxide can also be used for de-caffeinating coffee.
• Due to its cooling properties, CO₂ keeps food products cold while they are being transported. In addition, it can also be used for quick-freezing and, in combination with ethylene oxide, for cold sterilizing food.
• Furthermore, CO₂ is a highly effective inert blanket that protects food items during their production. It can displace air in the canning process, and it can be used to propel or extract food products from their containers.

In the related agricultural industry, there are some additional usages:

• In the grain treatment process, CO₂ is pumped into silos or other storage facilities to kill insects and protect the products. 
• Furthermore, the air in greenhouses can be enriched with carbon dioxide to allow the crops there to optimize their photosynthesis potential.

D. Metal Fabrication

Carbon dioxide is a shielding gas commonly used in metal fabrication ^[10]. It is a relatively inexpensive gas compared to other options like argon. It acts in a couple of ways:

• Protection: CO₂ creates a protective atmosphere around the weld pool, shielding it from contamination by oxygen and other gases in the surrounding air. This prevents oxidation, which can weaken the weld.
• Penetration: CO₂ contributes to deeper weld penetration due to its inherent properties affecting the arc characteristics.

II. EMERGING UTILIZATIONS OF CO₂

In this article the emerging utilization of CO₂ is divided into five main categories:

1. CO₂-derived fuel,
2. CO₂-derived chemicals,
3. Building materials from minerals and CO₂,
4. Building materials from waste and CO₂,
5. Miscellaneous research stage utilizations.

Whether a product can make a meaningful impact on carbon reduction depends on its scalability, competitiveness, and climate benefits. These factors determine its future market potential and whether it can make a measurable contribution to CO₂ reduction.

The IEA report ^[3] lists five key considerations in assessing the climate benefits of CO₂ use:

1. The source of CO₂ (from natural deposits, fossil fuels, biomass or the air)
2. The product or service the CO₂-based product or service is displacing.
3. How much and what form of energy is used to convert the CO₂.
4. How long the carbon is retained in the product.
5. The scale of the opportunity for CO₂ use.

The IEA report gives a good explanation of these factors. I just want to point out a couple of points that reflect what I mentioned at the beginning: 1. Cost of technology, input molecules, and energy requirement are important in each of the factors, 2. Retention time of CO₂ could make some CO₂ derived products more difficult to justify, 3. Some of the new utilization ideas may sound interesting but if the potential scale is small, it would have very little impact on carbon reduction. This scale factor is more limiting than the economic hurdles which could potentially be changed with government regulations or incentives. 

To understand the first two categories (CO₂-derived fuel and chemicals) it would be helpful if readers have a basic understanding of two main elements: syngas and Fischer-Tropsch process.

Syngas is simply a mixture of CO and H₂. To make hydrocarbons you need carbon and hydrogen. With syngas the CO molecule supplies the carbon and H₂ provides the hydrogen molecule. With this simple mixture a variety of hydrocarbon fuels and chemicals can be produced, and the most important production process is the Fischer-Tropsch process.

The Fischer–Tropsch process ^[11] is a collection of chemical reactions that converts syngas into liquid hydrocarbons, ideally having the formula (C_nH_2n+2). The more useful reactions produce alkanes as follows:

(2n+1) H₂ + nCO → C_nH_2n+2+nH₂O

Where n is typically 10–20. 

These reactions occur in the presence of metal catalysts, typically at temperatures of 150–300°C (302–572°F) and pressures of one to several tens of atmospheres. The Fischer–Tropsch process attracted a lot of research during the oil crisis (1970's-80's) when the supply and cost of petroleum was a major concern so coal gasification and gas-to- liquids technology were developed to produce liquid hydrocarbons. This process again receives significant interest today because of its potential to help the carbon reduction issue. It can be an important step of producing carbon-neutral liquid hydrocarbon fuels from CO₂ and hydrogen.

Figure 2, modified from ^[12], shows that a wide variety of products including diesel and gasoline can be produced via the Fischer-Tropsch synthesis process by using various catalysts and the feed stock is simple syngas.

Figure 2. Fischer Tropsch Synthesis with Syngas from Gasification or Captured CO₂ ^[12]

 

Where does the syngas come from? If you start with natural gas, coal, or biomass it must go through a gasification or reforming process to break down the hydrocarbons to hydrogen and CO. It is an energy intensive process.

Our interest here is to use captured CO₂ as the source of CO. As I mentioned before CO₂ is a very stable molecule, so it takes energy to convert it to the more reactive CO. The basic method is the reverse steam shift, but recent research proposes more energy efficient methods such as electrochemical conversion ^[14].

Figure 3 from ^[13] shows a broader spectrum of syngas usages. On the upper right area in red are the products from the Fisher-Tropsch process which are usually large molecules. The lower areas in yellow and orange are smaller molecule chemicals. They can be intermediate chemicals to make additional, more complex chemicals.

 

Figure3. Potential Usages of Syngas ^[13] 

 

One of the most useful intermediate chemicals is methanol (CH3OH). The two most significant developments after Fisher-Tropsch both use methanol as the starting molecule. They are the methanol-to-gasoline (MOG) and methanol-to-olefin (MOE) processes ^[13,15]. They will be described in more detail later. Similarly, ethanol-based processes have also been developed.

The areas on the left are other usages of syngas: power generation and H₂'s direct usages; they are outside the scope of this article.
From what was described above it seems that the pathways to use syngas and thus CO₂ are largely clear. Why are they still "emerging utilizations"? There are several reasons, and the most important one is the cost. In the sections below we will take a closer look at each of the utilizations.

 

A. CO₂-Derived Fuels

The carbon in CO₂ can be used to produce fuels that are in use today, including methane, methanol, gasoline and aviation fuels. CO₂-derived fuels are particularly interesting for applications where the use of other low-carbon energy carriers, such as electricity or hydrogen, is extremely challenging, such as in aviation. The aviation industry accounts for up to 3% of global CO₂ emissions so it is attracting a lot of attention.

Since fuel is supposed to be combusted and thus generates CO₂ so the retention of carbon in fuels is only temporal. The goal here is to create a circular economy in which no or at least less new fossil fuel is used. Airlines in recent years have expressed interest in exploring the potential. 

An example of commercialization is the company Emerging Fuels Technology (EFT) ^[16] which uses Fischer-Trosph process to make jet fuel, diesel and gasoline. EFT and its partner company, Twleve, have contracts with US air force and multiple airlines to supply the sustainable aviation fuel (SAF). Although the qualities of SAF have been verified, actual adoption of jet fuel by airlines has been slow. Today less than 1% of aviation fuel is SAF. Most of the companies are still in the exploratory stage due to the high cost of SAF.

Another example is the company Carbon One ^[17] which focuses on "green methanol". It advocates using green methanol as the best alternative fuel to the shipping industry. The company currently runs a pilot plant called Leuna 100. It is partially funded by the German government.

The competitive advantages of these companies are usually the catalysts they developed and the optimized process design to reduce capital and operating costs. Still, around 45–60% of the electricity used to produce CO₂-derived fuels is lost during the conversion processes ^[3].

As mentioned above methanol-to-gasoline (MTG) is another pathway to produce fuels from methanol. Methanol-to-gasoline chemistry was discovered by ExxonMobil scientists in the 1970s. The liquid product is conventional gasoline with virtually no sulfur and low benzene. MTG's reaction paths are depicted in Figure 4 ^[15]. ExxonMobil commercialized the first gas-to-gasoline plant in New Zealand in 1985. The New Zealand plant produces 14,500 barrels per day of gasoline. Recent developments of MTG are mainly in China where coal-to-gasoline is the main interest ^[3]. Basically, coal is gasified to make syngas and then syngas is converted to methanol, and finally, through the MTG process, to gasoline. Although the focus in China may not be CO₂ utilization at this point, it is conceivable that captured CO₂ could be an input gas to use the MTG technology.

 

Figure 4. Reaction Paths of ExxonMobil MTG Process ^[15] 

 

The company LanzaJet ^[18] uses ethanol as the starting molecule to produce SAF. A simplified flow diagram by the company is shown in Figure 5. Similar to the MTG process, ethanol is converted to light olefins (ethylene) first before being synthesized to SAF.

 

Figure 5. LanzaJet's SAF Production Process with Ethanol as Feed ^[18]

 

Examples of other CO₂-derived fuels that are less technologically mature are formic acid, dimethyl ether, ethanol and butanol, which can be used directly or as an intermediate to produce other fuels. Several novel conversion routes are being investigated, such as electrochemical conversion of CO₂ to CO ^[14] and direct reactions with the hydrogen (H₂) content of water in an integrated, single-step process. This pathway holds the promise of having lower capital costs than the technologically mature conversion routes. Other conversion routes include photochemical (use of sunlight) and biological processes (use of living organism such as enzymes). However, these processes are still in the early stages of their technological development ^[3].

 

B. CO₂-Derived Chemicals

 

B.1 Methanol and Methane

In the area of chemical intermediates, converting CO₂ to methanol and methane is the most technologically mature pathway. The 2020 report by CTCI Foundation (中技社) ^[19] provides a list of potential intermediate chemicals and their technical readiness. It then presented a good economic and environmental analysis of various processes that manufacture methane and methanol.

The general conclusion of the CTCI report is that the energy consumption of using CO₂ and green hydrogen to make methanol is about 2-3 times that of using natural gas. 90% of energy would be consumed in electrolyzing water.

 

B.2 Olefins

The most important intermediate chemicals that can be derived from methanol are olefins (e.g. ethylene, propylene), which are widely used in the production of polymers such as polyethylene (PE) and polypropylene (PP) to manufacture plastics. Methanol-to-olefins (MTO) technology is closely related to MTG. In fact, the intermediate products of the MTG reaction path, as shown in Fig. 4 are light olefins. 

The development in recent years was also in China with the main interest on coal-to-olefin. Again, although the focus in China is not CO₂ utilization it is conceivable that CO₂ could be an input gas to use the MTO technology. Methanol-to-olefins technology is currently deployed at commercial scale in China, accounting for 9 million tons per year (Mt/yr) or 18% of domestic high value chemicals production in 2018 ^[3].

In addition to MTO Figure 6 from ^[20] also shows additional development in the olefins area as well as the associated companies.

 

Figure 6. Methanol to Olefin Process Development ^[20]

 

Similar to MTO, the company, Technip Energies, has the ethanol-to-ethylene technology (acquired from BP Chemicals in 2016) called Hummingbird ^[21]. It is planning to build a plant in Texas with US Department of Energy's funding support to produce ethylene with CO₂ captured from the steam cracking process ^[22]. The first half of the LanzaJet SAF process depicted in Figure 5 is actually the Hummingbird process.

 

B.3 Polycarbonates

The most mature polymer manufacturing that uses CO₂ is the production of polycarbonate (PC). PC's annual production is over 4 million tons per year. It is transparent, has high impact resistance and high temperature resistance. So, it is widely used in a variety of consumer products, medical devices, automotives, and construction materials. 

Chimei Asai, a joint venture of Asahi Kasei Chemicals and Chi Mei Corp, has been operating a polycarbonate plant in Taiwan since 2002. It produces 150,000 tons of PC per year and was the first commercial plant to succeed in producing polycarbonates using CO₂ as a starting material.

Polycarbonate was conventionally produced by the reaction of bisphenol-A with either phosgene (carbonyl chloride COCl₂) or diphenyl carbonate. Phosgene is a poisonous agent and chloride can cause many operation difficulties. Asahi Kasei developed a new process ^[23] that uses CO₂ directly as a feed molecule. The key is the use of ethylene oxide (C₂H₄O) which reacts with CO₂ with catalyst to start a series of reactions. The reactions produce diphenyl carbonate as an intermediate product which then reacts with Bisphenol-A to make PC.

Reported emission reductions are 0.173-ton of CO₂ per ton of polycarbonate product compared to the conventional pathway. Figure 7 from Asahi's publication ^[23] illustrates the basics of this novel process. The Asahi paper provides more details about the process and its development history.

Since the Asahi invention more development has evolved. Germany's Covestro and Saudi Aramco's Novomer both developed CO₂-derived polycarbonate that contains 20-50% CO₂ by weight ^[3]. So PC production is one of the few very successful examples of carbon reduction and permanent carbon retention.

 

Figure 7 Reactions of Asahi Polycarbonate Process ^[23]

 

B.4 Polyurethane

Polyurethane (PU) is flexible and durable, and it is light weight. These properties make it suitable to make forms, coatings, and cushions. Global polyurethane production is over 23 million tons per year.

PU's main production mechanism is the reaction of polyols (molecules with multiple -OH groups) with poly-isocyanate (molecules with multiple -NCO groups).

Polyols can be made with CO₂ ^[23,24] in research. Here are the simplified main steps: 

1. CO₂ reacts with epoxide to form cyclic carbonates.
2. Cyclic carbonates then react with an initiator like diols to make polyols.

The Overall reaction (simplified) is as follows:

CO₂ + R-O-R' (epoxide) --> C₄H₆O₃ (cyclic carbonate) + initiator --> Polyol

Several companies are actively researching and developing CO₂-based polyol production technologies including BASF, Dow Chemical, and Covestro. Large-scale industrial production of PU is still in its early stages ^[25].

B.5 Chemicals with Carbonate Group (CO₃)

The amount of energy that must be added to convert CO₂ varies significantly per type of chemical. In general, producing chemicals from CO₂ that are rich in oxygen or contain a so-called carbonate group (CO₃) requires much less energy than producing olefins and paraffins (for example methane) that only contain carbon and hydrogen ^[3].

Examples of chemicals with a CO₃ group are sodium carbonate (soda ash) and sodium bicarbonate (baking soda). They are valuable chemicals for glass manufacture, cleaning agents and detergents and they can be manufactured from CO₂ and underground aqueous salt solutions (brine), seawater or salt (NaCl).

The chemical process often involves electrolysis to convert the salt-containing substance into a solution of sodium hydroxide. This solution is then reacted with CO₂ to produce soda ash or baking soda.

Several companies are active in using captured CO₂ to produce soda ash and baking soda today. One good example of commercialization is the company Carbon Free. Its SkyMine plant captures 50,000 tons of CO₂ per year from cement flue gas to transform it into sodium bicarbonate (baking soda) ^[26].

 

C. Building Materials (Concrete and Cement) from Minerals and CO₂

Cement is a fine powder, primarily consisting of limestone, clay, sand, and iron ore. It acts as a binder when mixed with water, forming a paste that hardens and binds other materials together. Cement is a key ingredient in concrete, mortar, and grout.

Concrete is a composite material composed of cement, aggregates (such as sand and gravel), and water. It is a versatile building material used for construction in various forms, including slabs, foundations, walls, and roads. When mixed with water, cement binds with the aggregates to form a solid and durable material through a chemical process called hydration.

In summary, cement is an ingredient used to make concrete, while concrete is the final product formed by mixing cement with aggregates and water. Concrete is a widely used construction material due to its strength, durability, and versatility.

With over 25 billion tons of concrete and 40 billion tons of mineral aggregates produced each year, the sheer size of the building materials market has led to mineral carbonation being recognized as one of the largest and most energy efficient ways to utilize CO₂.^[3]

It is considered as one of the most promising routes for carbon sequestration, with a $400 billion market opportunity and a potential to reduce annual CO₂ emissions by up to 3 Gt by 2030 ^[3].

Concrete is a mixture of cement, water and solid aggregates, such as sand, gravel and crushed stone. It can be produced as ready-mixed concrete which is transported in trucks and set on site, or as pre-cast concrete products. CO₂ can be used as a component of

1. the filler (aggregate), 
2. as a feedstock in the production of the binding material (cement),
3. as input for concrete curing.

All three applications are built around the same fundamental chemical process involving the reaction of CO₂ with minerals such as calcium oxide (burnt lime) or magnesium oxide (magnesia), to form carbonates. Carbonates is the form of carbon that makes up concrete ^[27]. The following are two of the many possible reactions that form calcium carbonate and magnesium carbonate.

        Ca₃SiO₅ + 3CO₂ + nH₂O→3CaCO₃ + SiO₂ • nH₂O

        Mg(OH)₂ + CO₂ + 2H₂O→MgCO₃ • 3H₂O

The converted carbonate can be used as a substitute for some of the limestone required in cement manufacturing. During this process, the previously captured CO₂ becomes chemically bound within the new cement product.

The biggest challenge of CO₂-derived building material is that the manufacturing of construction materials is a localized activity. If CO₂ is to be used in building materials on a largescale CO₂ needs to be consumed in many discrete locations and there are usually no large sources of CO₂ available in concrete plants. If CO₂ needs to be transported over long distance it would impact the economic viability of the manufactured product.

An example of commercialization is the Canadian company CarbonCure Technologies. CarbonCure developed a commercial CO₂ curing process that can be retrofitted to conventional "ready-mix" concrete plants. The process allows for the use of existing equipment and has little impact on the manufacturing conditions. On average, producers using CarbonCure Ready Mix reduce cement content by 3-5% with no compromise on concrete quality or performance. CarbonCure's customers can generate carbon removal credit for the concrete they make. The company claims that for every tonne of CO₂ used in CarbonCure concrete, around 254 tons of CO₂ can be avoided, mainly because less cement is needed per m3 of CO₂-cured concrete compared to conventionally produced concrete ^[27,28].

 

D. Building Materials from Waste and CO₂

Cement plays an important role in solidifying concrete after reacting with water, but a lot of CO₂ is emitted during manufacturing. Therefore, CO₂ emissions can be suppressed by using industrial by-products (blast furnace slag, coal ash, etc.) produced by steel plants and thermal power plants as substitute materials for cement.

CO₂ can be used to convert metal-containing waste materials into stable and solid carbonates with a market value. In many cases, the reaction products can be re-used in several applications, primarily in the construction industry as aggregates. Meanwhile, carbonation with CO₂ presents an opportunity to reduce the probability of the metals leaching and causing environmental harm, which may happen when stored in landfill or stockpiled on industrial sites, and to avoid costs associated with waste disposal. 

A wide variety of waste streams coming from the power or industrial sector could be technically remediated with CO₂, including coal fly ash, steel slag, cement-kiln dust, bauxite residue (red mud) and silicate mine tailings. Particularly alkaline wastes, such as fly ash and steel slag, make a good candidate due to their high concentration in reactive metals, such as calcium and magnesium ions (up to 40% by weight) ^[3].

A good example is the development of CO₂-SUICOM ^[29]. In 2011, Kajima Corporation, the Chugoku Electric Power Company, Denka Company Limited, and Landes Co., Ltd. jointly developed CO₂-SUICOM, the world's first type of concrete that reduces CO₂ emissions during manufacturing to virtually zero or less. CO₂-SUICOM is an acronym for "CO₂-Storage Utilization for Infrastructure by Concrete Materials." Compared to ordinary concrete, which emits 288 kg of CO₂ per cubic meter during production, CO₂-SUICOM cuts 306 kg of CO₂ per cubic meter by both absorbing and reducing CO₂. In other words, CO₂ emissions from concrete production will be virtually zero or less, and the more CO₂-SUICOM is made, the more CO₂ will be reduced ^[29].

Kajima further improved CO₂-SUICOM by using γ-C2S, a powdery substance with a property that reacts with CO₂ and hardens, and industrial byproducts from thermal power plants and steelworks as the main materials to absorb and harden large amounts of CO₂. Compared to conventional concrete, CO₂-SUICOM reduces the amount of cement by half. In addition, since CO₂-SUICOM fixes CO₂ when it hardens, it contributes to reducing CO₂. Fig 8 depicts how CO₂-SUICOM can be used.

CO₂-SUICOM is currently used in boundary blocks between sidewalks and roadways, road pavement blocks, river embankment blocks, and in balcony ceilings for apartment buildings ^[29].

Another example is the British company Carbon8 which uses around 5 kt/yr of CO₂ to convert around 60 kt/yr of air pollution control residues into lightweight aggregates as a component of building materials ^[30]. The climate benefits of these materials created from waste depend on the energy intensity of the production process and the transport of both the inputs and the carbonate products. Pretreatment and separation steps can be particularly energy intensive. The exact potential for reduction of emissions remains difficult to quantify and is case-specific. Carbon8 claims that more carbon is permanently stored during the process than emitted in its manufacture, resulting in a carbon-negative aggregate ^[30].

 

Figure 8: Input Components of CO₂-SUICOM ^[29] 

 

E. Miscellaneous Research Stage Utilization

There are many novel utilizations of CO₂ under research. The following is a small sample ^[3,31]. At this point they are at the early stages of research. They may be interesting and useful applications of CO₂ but the potential scale of CO₂ retention may not be large enough to merit serious consideration from the carbon reduction's point of view.

1. Production of novel materials such as carbon nanofibers (e.g. graphene).
2. Supercritical CO₂ as a green solvent to remove volatile organic compounds.
3. Use in Chemical Fluid Deposition (CFD) process.
4. Use as the expansion agent in polymers such as thermoplastic elastomer.

One area that deserves more discussion is using CO₂ to enhance the yields of biological processes. As mentioned before CO₂ has been used in industrial greenhouses to enrich the growing environment and thereby increase crop yields. However around 20% of the CO₂ fed to the greenhouses is absorbed by the crops, while the other 80% is vented with fresh air intake to control humidity. The absorbed CO₂ is fixed in the crops for a relatively short period until it is released into the atmosphere ^[3]. The benefit can only be claimed if the greenhouses used natural gas to supply the CO₂ before and thus natural gas can be displaced by captured CO₂.

A novel area that attracted a lot of attention 15 years ago was using CO₂ to grow genetically modified algae that could be used to produce diesel range fuels. Most notably Exxon Mobil announced a grand plan to do research in this area in 2007. Unfortunately, the challenges of low yields, high impurities, and high costs of processing the algal products prompted the company to quietly discontinue the research around 2016.

 

III. ASSESSMENT OF CARBON REDUCTION POTENTIAL

This section reports the high-level estimate of the carbon reduction potentials of the utilizations discussed in this article. The result is summarized in Figure 9. The assessment was conducted by IEA ^[3] with the author's own interpretation.

On the x-axis, the theoretical potential for CO₂ use refers to the maximum volumes of CO₂-derived products and services that would be generated if all conventionally produced products or services were to be replaced. The y-axis shows the relative climate benefits that can be achieved. CO₂ derived chemicals are divided into intermediates and polymers.

Fuels have the highest theoretical potential of over 5 Gt/yr because of the huge fuels volume in the world. However, their low retention time and high energy requirement to produce keep their climate benefit at the medium level.

Chemical intermediates, with 1-5Gt/yr potential, also have high energy requirements to produce. Polymers need less energy, but the potential volume (0-1 Gt/yr) is less.

Building materials have the greatest climate change mitigation potential, mainly because of their low energy requirements for the CO₂ conversion process and the permanent retention of carbon in the product.

 

Figure 9. Theoretical Potential and Climate Benefits of CO₂-derived products and Services ^[3]

 

IV. CONCLUDING REMARKS

The market for CO₂ utilization is expected to remain relatively small in the short term but has the potential to grow in the long term. Several measures are recommended as follows:

• Increasing government subsidies and/or regulation would increase the adoption of CO₂-derived products. This is already happening in the US and EU to some extent. For example, EU sets a schedule to gradually increase the percentage of SAF in aviation fuels. US's Department of Energy just distributed 6 billion dollars to encourage companies to invest in products with captured CO₂ emissions ^[22]. In Taiwan similar incentives may need to be created for companies to adopt the higher cost CO₂-derived products. 
• One major hurdle to producing fuels and chemical intermediates from CO₂ is the hydrogen production cost. In the intermediate term the cost could be reduced by using blue hydrogen which is a combination of steam methane refining with CCS. In the longer term the energy required to electrolyze water needs to be reduced significantly. Continued research in the efficiency of electrolysis is needed. In Taiwan CPC just started a CCS pilot project in 2023. If CCS is successful, it would open the door to not only sequestration of existing CO₂ emissions but also blue hydrogen production for the petrochemical industry.
• The other part of syngas, CO, also needs research to reduce the energy requirement to convert from CO₂. A very recent publication ^[32] shows that there is a lot of room for innovation. I believe Taiwan's researchers can contribute to this area.
• Building materials demonstrate great carbon reduction potential. However reliable long-term performance, tests in heavy-load markets, and broader acceptance for these products are needed. So multi-year trials and close collaboration between government and industry are needed to update and extend existing product standards and building codes. Given its potential and relatively low energy requirement Taiwan probably should start research and trial activities in this area. The potential market in the world could be very large.

 

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