It would require approximately 415% (4.15x) of the total electrical energy generated in the United States in 2019 to counter the global emissions from all global hard-to-abate sectors based on current direct air capture efficiencies (not including storage and capital energy requirements). However, this value would drop significantly as the hard-to-abate sectors achieve large scale decarbonization

Summary

Hard-to-abate sectors like heavy industry, shipping, and cement production account for 9.6 – 11.6 gigatons of annual CO₂ emissions. These industries face unique decarbonization challenges due to high heat requirements, fossil fuel feedstock requirements, and chemical processes that inherently produce CO₂. While electrification and alternative fuels offer partial solutions, many sectors will still struggle to decarbonize. Direct air capture (DAC) could theoretically offset these emissions but would demand enormous energy inputs - at current efficiencies, countering all hard-to-abate emissions would require 414.5% of the United States' 2019 electricity generation. The energy requirement drops to 32.3% with perfect 100% efficiency that is not achievable, highlighting the critical need for both DAC efficiency improvements and sectoral decarbonization efforts.

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Hard to abate sectors are industries and processes that are difficult to decarbonize. After global carbon dioxide (CO₂) emissions have been stabilized, direct air capture (DAC) could be used to remove the CO₂ produced from these sectors. Industries can be considered hard to abate due to a number of factors such as high energy requirements, specialized energy requirements such as high temperatures, or processes that naturally produce large amounts of CO₂ emissions such as the production of cement.

Abatement for hard to abate sectors could be achieved through the use of direct air capture (DAC) technology.

Table 1: Annual CO₂ emissions from hard-to-abate sectors

Sector	Annual Emissions [Gt CO₂] 1 2 3
Heavy-duty trucks	1.8
Shipping	0.82 – 0.86
Aviation	0.58 – 0.83
Iron and Steel	2.7 – 3.6
Chemicals and Petrochemicals	1.3 – 1.7
Ammonia (NH₃)	0.42 – 0.55
Methanol	0.26 – 0.34
High-value chemicals (HVCs)	0.25 – 0.33
Concrete (cement and lime)	2.4 – 2.8
Total	9.6 – 11.6

Heavy-duty trucks

Heavy-duty trucking accounts for close to 25% of all CO₂ emissions originating from the transportation sector, or approximately 5% of all CO₂ emissions globally ¹. It is estimated that the CO₂ emissions from heavy-duty trucking are greater than those from the aviation and shipping industries combined ¹. The trucking industry has recently been undergoing a slow transition to higher efficiency engines, but with the continued growth and reliance on the industry, emissions are expected to almost double from 1.8 gigatons (Gt) in 2022 to 3 gigatons (Gt) of CO₂ by 2050 ¹.

The trucking industry is well primed for a transition to full electrification; however, as with all energy transitions, this will be a slow process and will take many decades. Of all the identified hard-to-abate sectors, heavy-duty trucking will be one of the least likely to rely on offsetting by direct air capture, due to the relative ease of electrification and the fact that many heavy-duty truck manufacturers have already expanded production of electric vehicles.

Shipping

In 2022, international shipping accounted for between 2% and 3% of all global CO₂ emissions, or 0.82 - 0.86 Gt of CO₂ ^{1 3}. Maritime shipping accounts for over 80% of all global trade by volume, and close to a third of that figure is purely for the transportation of fossil fuel products ¹. Although maritime shipping is considered one of the least carbon intensive forms of transport per volume of cargo, the combined emissions of the industry are still significant. Additionally, the International Maritime Organization (IMO) expects that international shipping activity will grow by 40%-100% by 2050 ⁴.

Electrification of the shipping industry is possible; however, it is progressing at a much slower pace compared to land-based electrified transport, with only two existing container ships capable of traveling up to 1000 km in operation in 2023 ¹.

The maritime shipping industry is a prime example of a sector that can benefit from direct air capture technology in the long term. Container ships are already capable of using synthetic fuels which require a large feedstock of carbon dioxide that can be provided by direct air capture.

Aviation

Aviation is considered as one of the most carbon intensive forms of transport, accounting for between 2% and 3% of all global CO₂ emissions, around 0.58 – 0.8 Gt of CO₂ per year as of 2022 ^{1 3}. The aviation industry is also projected to grow to almost double by the year 2050 ¹. Because energy costs are a large part of the cost of flying and are where airlines are able to lower costs, large improvements in efficiency have already been made. The key challenge to decarbonizing aviation is the requirement for highly energy dense fuels to keep volume and weight down ¹.

With the rapid advancements currently seen in battery technology, electrification may be possible for hybrid and short-distance aircraft, but distances are still limited by the energy density of batteries. Biofuels are seen as a promising solution for replacing fossil fuels; however, as with all biofuel-related solutions, there are concerns over the sustainability of the feedstock and the potential for competition with food production ¹.

Synthetic fuels such as e-kerosene are a promising option for the decarbonization of aviation as it has a high energy density and can be used directly with aircraft that are currently in operation. The production of e-kerosene requires a large feedstock of CO₂, which can be provided by direct air capture ¹.

Iron and Steel

Steel is one of the most important materials in the world and is used in a wide range of products. Current annual demand for steel is around 1,850 Mt and is expected to grow to 2,500 Mt by 2050 ¹. Currently, the steel industry accounts for approximately 7% of all global emissions or between 2.7 - 3.6 Gt of CO₂ per year as of 2022, making it the largest segment of the hard-to-abate sectors ^{1 2 3}.

Steel is produced through either primary production, where iron ore is reduced to iron and then converted to steel, or through steel recycling which is known as secondary production. Primary production of steel is highly energy intensive and carbon intensive, utilizing metallurgic coke (high quality coal) to reduce iron ore into iron. The usage of coke is important as it provides heat for the reaction, but also carbon monoxide (CO) which is vital for the reduction process. Secondary production is much more energy efficient and carbon efficient, where scrap steel is melted down using enormous electric arc furnaces which can theoretically be carbon neutral depending on the electricity source.

Increased penetration of secondary production of steel is expected to be the main form of decarbonization of the industry as the feedstocks of available scrap steel continue to increase in line with recent trends in steel production, and it is expected that half of all steel production will come from secondary sources by 2050 ¹.

Decarbonization of primary steel production is a greater challenge due to the cost effectiveness and efficiency of the blast furnace process. A process known as hydrogen-based direct iron reduction (DRI) which utilizes hydrogen as the main reduction agent is a promising technology that is almost at a state of commercial maturity. It is estimated that 40% of new steel production projects currently in the planning stage are using DRI technology ¹. However, hydrogen DRI still has a number of barriers such as green hydrogen sourcing and infrastructure, and the requirement for high grade iron ore.

Due to the scale of steel production and the challenges faced in decarbonizing primary production, direct air capture could play a role in offsetting emissions from steel production.

Chemicals and petrochemicals

The chemical and petrochemical industry is considered a hard-to-abate sector as many important chemicals are derived from fossil fuel feedstocks and require high energy and heat inputs. Currently, the chemicals industry accounts for 4% of global CO₂ emissions or approximately 1.3 - 1.7 Gt of CO₂ annually, and this figure is expected to increase to 4.7 Gt by 2050 ^{1 2}. The majority of the chemical industry's CO₂ emissions stem from the production of ammonia, methanol, and a class of chemicals known as high-value chemicals (HVCs). Approximately 72% of all CO₂ emissions stemming from the chemicals industry are produced from the production of ammonia (32.4%), methanol (20.16%), and HVCs (19.4%) ¹.

Ammonia (NH₃)

Ammonia is an important primary chemical as it is mainly used in the synthesis of nitrogen fertilizer which is vital for maintaining food security. Ammonia is carbon intensive because it requires a large feedstock for its synthesis, which is usually derived from natural gas, i.e., methane (CH₄) through a process known as Steam Methane Reforming (SMR), but it can also be sourced from coal.

First, the methane is reacted with steam at high temperatures (~900°C) to produce hydrogen (H₂) and carbon monoxide (CO)

$$\begin{align*} \mathrm{CH_4 + H_2O \rightarrow CO + 3H_2} \end{align*}$$

Then, the carbon monoxide is reacted again via the Water-Gas Shift Reaction to produce more hydrogen and carbon dioxide as a byproduct

$$\begin{align*} \mathrm{CO + H_2O \rightarrow CO_2 + H_2} \end{align*}$$

Generally, the CO₂ that is emitted at the end of the water gas shift reaction is emitted directly to the atmosphere. Natural gas is also usually utilized as the fuel source for steam generation and it provides the heat required for the reactions, adding even more emissions ¹

Decarbonization of the ammonia production process can be achieved through the usage of hydrogen that is sourced from sustainable methods such as electrolysis of water. However, the price of 'renewable' ammonia currently sits at a range of $720 - $1400 per ton, around three times the price of conventional fossil fuel derived ammonia at $110 - $610 per ton ¹. The price of renewable hydrogen is expected to decrease down to $310 - $610 per ton in 2050, still higher than the conventional method. Because of this, direct air capture could be utilized to offset the emissions produced from the ammonia production process.

Methanol (CH₃OH)

Methanol is a critical chemical used in the production of pharmaceuticals, solvents, plastics, and other chemicals. Similar to ammonia, the synthesis of methanol requires hydrogen molecules which are extracted from natural gas using the steam methane reforming (SMR) process as detailed in the above ammonia section. The SMR process is the most carbon-intensive step of methanol production, and as of 2021, 65% of all global methanol production was sourced from natural gas, 35% from coal, and just 0.2% from renewable sources such as green hydrogen ¹. Once hydrogen is sourced, it can be reacted with carbon dioxide (CO₂) or carbon monoxide (CO) in the presence of a catalyst and specific ambient conditions to produce methanol via the following reactions:

$$\begin{align*} \mathrm{CO_2 + 3H_2 \rightarrow CH_3OH + H_2O} \end{align*}$$ $$\begin{align*} \mathrm{CO + 2H_2 \rightarrow CH_3OH} \end{align*}$$

Decarbonization of the methanol production process can be achieved primarily through avoiding the use of methane and the steam methane reforming process by using renewable hydrogen in the same way as decarbonization of ammonia. Methanol can also be created from biomass (biomethanol), which involves the fermentation of organic feedstock to produce methanol. The key differences in the decarbonization of ammonia is that methanol production also requires a sustainable feedstock of carbon dioxide (CO₂) or carbon monoxide (CO) which can be provided by direct air capture ¹.

High-value chemicals (HVCs)

High-value chemicals (HVCs) are a class of important industrial chemicals that are typically derived from fossil fuel feedstocks and have significant economic value as they are the building blocks for a wide range of products such as plastics ¹. Some important HVCs include ethylene, propylene, and aromatics such as benzene.

Biomass feedstock is a potential solution for the replacement of fossil fuel feedstock which can yield biodegradable plastics; however, the cost of bioplastics is a major barrier for widespread adoption as the bioplastic industry is still considered at a low level of maturity. Bio-based feedstocks are another option for decarbonization, such as the synthesis of bio-based ethylene which is chemically identical to ethylene derived from petroleum ¹.

Hydrocarbons can also be synthesized from renewable hydrogen and a feedstock of carbon dioxide (CO₂) to be used as a replacement for fossil fuel derived HVCs. However, the cost of these processes is significantly high compared to the bio-based alternatives. With the mass adoption of direct air capture, the cost of these processes is expected to decrease significantly, allowing for the production of HVCs to become more cost competitive with bio-based and fossil fuel derived HVCs ¹.

Concrete (cement and lime)

Cement production is the second largest individual source of industrial CO₂ emissions behind the iron and steel industry, accounting for approximately 2.8% (2.4 - 2.8 Gt) of all global CO₂ emissions ^{2 3}. Cement mainly consists of calcium oxide (CaO), silicon oxide (SiO), and smaller parts of aluminum oxide (Al₂O₃) and iron oxide (Fe₂O₃). Annually, around 4 billion tons of cement is produced through the process of calcination, which is the main source of CO₂ emissions.

Calcination is the process used for the production of calcium oxide, where limestone (CaCO₃) is heated to high temperatures (~1400°C) to produce calcium oxide (CaO) and carbon dioxide (CO₂) as a byproduct.

$$\begin{align*} \mathrm{CaCO_3 + (heat) \rightarrow CaO + CO_2} \end{align*}$$

In order for this reaction to occur, high temperatures over 800°C (1500°F) are required. The conventional method for achieving this is through the use of a large rotary kiln which facilitates a continuous process of calcination. As with most processes that require large amounts of high temperature heat, fossil fuels are traditionally used. Therefore, it can be seen that the cement production process creates CO₂ emissions from two sources, the first being the by product of the calcination reaction (see in the equation above), and the second being the combustion of fossil fuels. For every kg of cement produced, approximately 0.6 kg of CO₂ is emitted to the atmosphere ².

Decarbonization of cement production could involve the use of alternative fuels such as green hydrogen or biofuels to provide the heat required for the calcination reaction. Additionally, electrification is an option which bypasses the need for fossil fuels. Currently, there are a few companies such as Sublime Systems that have developed electrochemical processes for the production of cement; however, these are still in their pilot stages and would need to be scaled up enormously to meet the global demand for cement ².

The key challenge for decarbonization of cement production is the fact that the main feedstock of limestone (CaCO₃) contains embedded carbon. Because of this, decarbonization methods would require the use of limestone alternatives that contain less carbon, such as calcium silicate rocks ². Because of this challenge, direct air capture could play a key role in the decarbonization of future cement production through offsetting the CO₂ emissions from the calcination reaction.

Energy requirements for direct air capture

Many of the hard-to-abate sectors will eventually transition to low or zero carbon operations in the future, such as with the heavy-duty trucking industry which is already undergoing a transition to electrification. However, even with these transitions, the hard-to-abate sectors will still produce large amounts of CO₂ emissions. It is possible to calculate the theoretical minimum energy required to capture CO₂ depending on the ambient concentration of CO₂ in the atmosphere. Previously we calculated that it would take a minimum of approximately 420 kJ of energy to capture 1 kg of CO₂ from the atmosphere (assuming an unrealistic 100% efficient capture process). Using this value, we can calculate the minimum energy required to capture the CO₂ emissions from the hard-to-abate sectors.

Using the upper estimate of CO₂ emissions from the hard-to-abate sectors taken on table 1, we can estimate the total emissions to be around 11.6 Gt of CO₂ per year as shown in table 2.

We can calculate the energy requirement from the following equation:

$$\begin{align*} \text{Energy}_{capture} &= CO_{2\text{\_removed}} \space \text{[Gt]} \times 420 \space \text{[kJ/kg]} \times \frac{1}{\eta} \\ \end{align*}$$

Where $\eta$ is the efficiency of the capture process:

• $\eta = 1$ for 100% efficient capture process
• $\eta = 0.5$ for a 50% efficient capture process
• $\eta = 0.078$ for a 7.8% efficient capture process (realistic efficiency)

Table 2: Energy requirements for direct air capture of CO₂ from the hard-to-abate sectors

Efficiency ($\eta$)	Energy requirement for capture [EJ]	Percentage of total electricity generated in the United States in 2019 5
100%	4.87	32.3%
50%	9.74	64.6%
7.8% (realistic)	62.46	414.5%

For a comparison in the United States in 2019, total energy supplied (including energy generation and the use of fossil fuels in industry and transport) was equal to 92.64 EJ, while the electricity generation alone was equal to 4,186 TWh or 15.07 EJ ⁵. This would mean that a significant portion of the energy generated in the United States in 2019 would need to be diverted to direct air capture to offset the CO₂ emissions from the hard-to-abate sectors as they stand now without any decarbonization.

Sources

Footnotes

1. International Renewable Energy Agency. (2024). Decarbonising hard-to-abate sectors with renewables: Perspectives for the G7. Abu Dhabi, United Arab Emirates. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶ ↩¹⁷ ↩¹⁸ ↩¹⁹ ↩²⁰ ↩²¹ ↩²² ↩²³ ↩²⁴ ↩²⁵

2. Buznitsky, K., Verma, S., & Nitzsche, M. P. (2024). Decarbonizing industry: Policy approaches to eliminate hard-to-abate emissions. MIT Science Policy Review. https://sciencepolicyreview.pubpub.org/pub/3v3e6ed7. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷

3. United Nations Industrial Development Organization. (2023). Policy packages for decarbonizing heavy industry (IID Policy Brief No. 9). https://www.unido.org/sites/default/files/unido-publications/2023-12/IID%20Policy%20Brief%209%20-%20Policy%20packages%20for%20decarbonizing%20heavy%20industry.pdf. ↩ ↩² ↩³ ↩⁴ ↩⁵

4. International Maritime Organization. (2021). Fourth IMO greenhouse gas study 2020. https://www.imo.org/en/OurWork/Environment/Pages/Fourth-IMO-Greenhouse-Gas-Study-2020.aspx (accessed 10 February 2024). ↩

5. International Energy Agency. (2021). Key world energy statistics 2021. IEA. https://www.iea.org/reports/key-world-energy-statistics-2021. ↩ ↩²