Carbon capture and utilization

Carbon capture and utilization (CCU) is the process of capturing carbon dioxide (C O₂) to be recycled for further usage.[1] Carbon capture and utilization may offer a response to the global challenge of significantly reducing greenhouse gas emissions from major stationary (industrial) emitters. CCU differs from Carbon Capture and Storage (CCS) in that CCU does not aim nor result in permanent geological storage of carbon dioxide. Instead, CCU aims to use the captured carbon dioxide for conversion into other substances or products with higher economic value (e.g. plastics, concrete, biofuel) while retaining the carbon neutrality of the production processes.

Comparison between sequestration and utilization processes of captured carbon dioxide

There are several technologies available in CCU, such as converting CO₂ to products such as methanol, biofuel, and other forms of hydrocarbons for use as alternative and renewable sources of energy. Other forms of CCU includes the conversion of CO₂ into commercial products such as plastics, concrete and reactants for various chemical synthesis.[2]

Although CCU does not result in a net carbon positive to the atmosphere, there are several important considerations to be taken into account. The energy requirement for the additional processing of new products should not exceed the amount of energy released from burning fuel as the process will require more fuel. Manufacturing products from CO₂ is energy intensive as the CO₂ is a thermodynamically stable form of carbon.[3] In addition, concerns on the scale of CCU is a major argument against investing in CCU. The availability of other raw materials to create a product should also be considered before investing in CCU.

One of the drivers for the possible implementation of CCU is a price on carbon. A price on carbon will incentivize the reduction of CO₂ being released into the atmosphere. Thus, CCU can be one of the main alternatives for a company to reuse the emitted CO₂ for creating useful commercial products.[4]

Sources of carbon

CO₂ is typically captured from fixed point sources such as power plants and factories.[3] CO₂ captured from these exhaust stream itself varies in concentration. A typical coal power plant will have 10-12% CO₂ concentration in its flue gas exhaust stream.[5] A biofuel refinery produces a high purity (99%) of CO₂ with small amount of impurities such as water and ethanol.[5] The separation process itself can be performed through separation processes such as absorption, adsorption, or membranes.

Another possible source of capture in CCU process involves the use of plantation. The idea originates from the observation in the Keeling curve that the CO₂ level in the atmosphere undergoes annual variation of approximately 5 ppm (parts per million), which is attributed to the seasonal change of vegetation and difference in land mass between the northern and southern hemisphere.[6][7] However, the CO₂ sequestered by the plants will be returned to the atmosphere when the plants died. Thus, it is proposed to plant crops with C4 photosynthesis, given its rapid growth and high carbon capture rate, and then to process the biomass for applications such as biochar that will be stored in the soil permanently.[8]

Examples of technology and application

Carbon-neutral fuel

A carbon-neutral fuel can be synthesized by using the captured CO₂ from the atmosphere as the main hydrocarbon source. The fuel is then combusted and CO₂, as the byproduct of the combustion process, is released back into the air. In this process, there is no net carbon dioxide released or removed from the atmosphere, hence the name carbon-neutral fuel. An example of the technology include biofuel from microalgae as discussed below.

Methanol fuel

Methanol, or methyl alcohol, is the simplest member of the family of alcohol organic compound with a chemical formula of C H₃O H. Methanol fuel can be manufactured using the captured carbon dioxide while performing the production with renewable energy. Consequently, methanol fuel has been considered as an alternative to fossil fuels in power generation for achieving a carbon-neutral sustainability.[9][10] Carbon Recycling International, a company with production facility in Grindavik, Iceland, markets such Emission-to-Liquid renewable high octane methanol fuel with current 4,000 metric ton/year production capacity.[11]

Chemical synthesis

Also known as chemical feedstock, CO₂ captured previously will be used to be converted to a diverse range of products. Some of these products include: polycarbonates (via Zinc based catalyst) or other organic products such as acetic acid,[12] urea,[12] and PVC.[13] A March 2011 report suggested that this technology requires 1–5 years to commercialization. Chemical synthesis is not a permanent storage/utilization of CO₂, as aliphatic (straight chain) compounds may degrade and release CO₂ back to the atmosphere as early as 6 months.[13]

Industrial example of chemical synthesis: Novomer

Novomer is a chemicals company working in the synthesis of everyday products. They are working in two different types of plastics: polyethylene carbonate (PEC) and polypropylene carbonate (PPC) and claim that their products contains up to 50% CO₂ by mass. A March 2011 report by Global CCS Institute foresaw annual production potential of 22.5 MtCO₂/yr.[13] They have received funding from multiple sources such as the Department of Energy (DOE) ($2.6 million) and NSF ($400,000) to achieve commercialization as well as converting their production process from a batch process to a continuous process. Their pilot plant opened in December 2009 with a 1500L batch reactor and utilized waste stream from ethanol fermentation and flue gas as their source of CO₂.

The advantage of chemical synthesis by Novomer is that there is a diverse choice for characteristics of plastics being made. This helps avoid the scarcity of resources if this technology is to be scaled up. Another advantage of this technology is that the CO₂ sources won't compete with food production.

One disadvantage of this technology is that the technology is relatively new, and more research is required into synthesis components such as the zinc-based catalyst. Another is that its choice of CO₂ source may give rise to the need for another separation process to increase CO₂ quality. A big problem present in Novomer's case is the packaging industry, which prioritises low cost polymer, which CO₂-based polymers would have to compete against.

Enhanced Oil Recovery (EOR)

In EOR, the captured CO₂ is injected into depleted oil fields with the goal to increase the amount of oil to be extracted by the wells. This method is proven to increase oil output by 5-40%.[13] The scale of CO₂ utilization through this technologies ranges from 30-300 MtCO₂/yr. It is a permanent and mature technology in CCU. The biggest market driver for EOR is the heavy reliance on oil. In United States, some of the additional market drivers include: tax revenue for foreign oil as well as the presence of carbon tax credits.

Carbon mineralization

Carbon dioxide from sources such as flue gas are reacted with minerals such as magnesium oxide and calcium oxide to form stable solid carbonates. Sources of minerals include brine and waste industrial minerals. The carbonates can then be used for construction, consumer products, and as an alternative for carbon capture and sequestration (CCS). The scale of this technology may reach more than 300 Mt of CO₂ removed per year. 0.5 tonnes of CO₂ is removed from the air for every tonne of mineral carbonate produced. However, it needs 1–5 years to commercialization as the technology is not mature yet.

The company Calera proposed a way to mineralize CO₂ through the CMAP process. This process involves precipitating a carbonate slurry from a mixture of water, solid minerals, and flue gas. The products are concentrated pumpable carbonate suspension, fresh water, and CO₂-free flue gas.

Benefits of this process includes the production of fresh water and that the CO₂ used does not require separation or compression. A barrier of this technology is, however, the competition with existing cement industries.

Biofuel from microalgae

Fuels can be produced from algae

A study has suggested that microalgae can be used as an alternative source of energy.[14] A pond of microalgae is fed with a source of carbon dioxide such as flue gas and the microalgae is then let to proliferate. The algae is then harvested and the biomass obtained is then converted to biofuel. 1.8 tonnes of CO₂ is removed from the air per 1 metric tonne of dry algal biomass produced. This number actually varies depending on the species. The scale of this technology may reach more than 300Mt of CO₂ removed per year. The CO₂ captured will be stored non-permanently as the biofuel produced will then be combusted and the CO₂ will be released back into the air. However, the CO₂ released was first captured from the atmosphere and releasing it back into the air makes the fuel a carbon-neutral fuel. This technology is not mature yet.[15]

Dead algae can sink to the bottom of the lake and result in permanent storage. However, the algae needs a large area of pond and sunlight all year round to remove CO₂ all year round. Furthermore, the pond environment needs to be controlled since the algae needs to live in a specific condition. There are concerns about how the algae-filled pond might affect the environment and ecosystem around it.

Agriculture

An approach that is also proposed as a climate change mitigation effort is to perform plant-based carbon capture.[16] The resulting biomass can then be used for fuel, while the biochar byproduct is then utilized for applications in agriculture as soil-enhancer. Cool Planet is a private company with an R&D plant in Camarillo, California, performed development of biochar for agricultural applications and claimed that their product can increase crops yield by 12.3% and three-fold return of investment via improvement of soil health and nutrient retention.[17] However, the claims on the efficacy of plant-based carbon capture for climate change mitigation has received a fair amount of skepticism.[18]

Environmental impacts

Sites of Carbon Capture and Utilization projects and development, per 2011 report from Global CCS Institute.[19]

16 life cycle environmental impact analyses have been done to assess the impacts of four main CCU technologies against conventional CCS: Chemical synthesis, carbon mineralization, biodiesel production, as well as Enhanced Oil Recovery (EOR). These technologies were assessed based on 10 Life-cycle assessment (LCA) impacts such as: acidification potential, eutrophication potential, global warming potential, and ozone depletion potential. The conclusion from the 16 different models was that chemical synthesis has the highest global warming potential (216 times that of CCS) while enhanced oil recovery has the least global warming potential (1.8 times that of CCS).[1]

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References

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