Emissions from hard-to-abate sectors after widespread global decarbonization could still total around 4.86-6.82 Gt of CO₂ per year, potentially growing to 9.72-13.64 Gt CO₂ if consumption doubles.

Summary

Residual CO₂ emissions from critical industries persist even after aggressive decarbonization efforts, particularly in sectors requiring direct air capture (DAC) solutions. Heavy transport, steel production, chemicals manufacturing, and cement/concrete could emit 4.86-6.82 Gt CO₂ annually under Net Zero Emissions scenarios, potentially doubling with rising demand. Adopting hydrogen-based pathways reduces these emissions to 3.50-4.87 Gt CO₂ annually. DAC systems would require 18.85-36.72 EJ of energy annually at 7.8% efficiency, alongside massive material inputs including millions of tonnes of ethanolamine and sulfuric acid for sorbent production. While hydrogen enables full decarbonization in shipping and ammonia production, aviation and cement industries will likely depend on permanent carbon cycling solutions. Detailed emissions projections and resource requirements appear in Tables 1-4, based on 2022 data and IEA's Net Zero pathway assumptions.

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After a rapid and aggressive decarbonization of the global economy that is required to stabilize atmospheric carbon dioxide (CO₂) concentrations to a level that is sustainable for continued human existence, many of the sectors currently considered "hard-to-abate" will undergo high degrees of decarbonization. However, even after this shift, there will still be industries that are simply unable to decarbonize due to issues such as material feedstock requirements and other process difficulties. These remaining sectors could rely on direct air capture (DAC) to remove CO₂ from the atmosphere to facilitate the continued operation of vital hard-to-abate industries.

Many of the gains of decarbonizing hard-to-abate sectors can only be achieved through the creation of a hydrogen economy with a focus on green hydrogen, as hydrogen serves as the main feedstock in many of the hard-to-abate sectors, and has potential applications in steel production and as an alternative fuel source. Therefore it will also be necessary to analyze any future scenarios both with and without the widespread adoption of hydrogen.

Note: The estimates for remaining emissions from hard-to-abate sectors are based on the assumption that full practical decarbonization has been achieved after a sufficiently long timeframe, meaning all outdated CO₂-emitting technologies replaceable by electrification have been replaced. These figures are based on 2022-2023 emissions estimates and do not account for future projections, which will most likely exceed current estimates as consumption increases.

Table 1: Annual CO₂ emissions from hard-to-abate sectors. Values in parenthesis represent a doubling of 2022 emissions levels

Sector	Current Emissions [Gt CO₂]	Net Zero Emissions Scenario [Gt CO₂]	NZE with Maximum Hydrogen Penetration [Gt CO₂]
Heavy-duty trucks	1.8	0.2 - 0.45 (0.4 - 0.9)	0.20 - 0.45 (0.4 - 0.9)
Shipping	0.82–0.86	0.11 - 0.30 (0.22 - 0.6)	0 (0)
Aviation	0.58–0.83	0.2 - 0.46 (0.4 - 0.92)	0.20 - 0.46 (0.4 - 0.92)
Iron and Steel	2.7–3.6	1.79 - 2.48 (3.58 - 4.96)	1.12 - 1.57 (2.24 - 3.14)
Chemicals and Petrochemicals	1.3–1.7	0.82 - 1.10 (1.64 - 2.2)	0.42 - 0.57 (0.84 - 1.14)
Ammonia (NH₃)	0.42-0.55	0.27 - 0.36 (0.54 - 0.72)	0 (0)
Methanol (CH₃OH)	0.26-0.34	0.20 - 0.26 (0.4 - 0.52)	0.07 - 0.09 (0.14 - 0.18)
High-value chemicals (HVCs)	0.25-0.33	0.20 - 0.26 (0.4 - 0.52)	0.20 - 0.26 (0.4 - 0.52)
Concrete (cement and lime)	2.4–2.8	1.74 - 2.03 (3.48 - 4.06)	1.56 - 1.82 (3.12 - 3.64)
Total	9.6-11.6	4.86 - 6.82 (9.72 - 13.64)	3.50 - 4.87 (7.0 - 9.74)

Table 2: Idealized capture energy for CO₂ from air assuming 7.8% thermal efficiency. Values in parenthesis represent a doubling of 2022 emissions levels

Sector	Net Zero Emissions Scenario [EJ]	NZE with Maximum Hydrogen Penetration [EJ]
Heavy-duty trucks	1.08 - 2.42 (2.15 - 4.85)	1.08 - 2.42 (2.15 - 4.85)
Shipping	0.59 - 1.62 (1.18 - 3.23)	0 (0)
Aviation	1.08 - 2.48 (2.15 - 4.95)	1.08 - 2.48 (2.15 - 4.95)
Iron and Steel	9.64 - 13.35 (19.28 - 26.70)	6.03 - 8.45 (12.06 - 16.90)
Chemicals and Petrochemicals	4.42 - 5.92 (8.83 - 11.85)	2.26 - 3.07 (4.52 - 6.14)
Ammonia (NH₃)	1.45 - 1.94 (2.91 - 3.88)	0 (0)
Methanol (CH₃OH)	1.08 - 1.40 (2.15 - 2.80)	0.38 - 0.48 (0.75 - 0.96)
High-value chemicals (HVCs)	1.08 - 1.40 (2.15 - 2.80)	1.08 - 1.40 (2.15 - 2.80)
Concrete (cement and lime)	9.37 - 10.93 (18.74 - 21.86)	8.40 - 9.80 (16.80 - 19.60)
Total	26.18 - 36.72 (52.34 - 73.44)	18.85 - 26.22 (37.70 - 52.44)

Decarbonization: Net Zero Pathway and Hydrogen Pathway

When we think of decarbonization, we often think of rapid reductions in CO₂ emissions sourced from fossil fuels, accompanied by large scale electrification and the adoption of sustainable fuels where necessary. This is what we will refer to as the "Net Zero Emission" scenario of decarbonization, as it is the most familiar form of decarbonization and it has been the process by which the world has attempted over the past few decades. The Net Zero Emission scenario is assisted by the fact that infrastructure for electrification has existed for almost a century, and electrification of regular and hard-to-abate sectors has been underway for some time. The estimates for the decarbonization pathway to achieve net zero are based on the International Energy Agency's (IEA) Net Zero Emissions by 2050 pathway from the 2022 world energy outlook. They describe the net zero emissions pathway as follows:

"The NZE Scenario is based on the deployment of a wide portfolio of clean energy technologies, with decisions about deployment driven by costs, technology maturity, market conditions and policy preferences. The pathway reflects the particular circumstances of various countries in terms of resource and infrastructure endowments, development pathways and policy preferences." ¹

The IEA's Net Zero Emission scenario is just one of a number of possible pathways that can be taken to achieve global decarbonization. Although it is used by the IEA to describe a pathway for net zero emissions by 2050, it will be utilized here to describe a generalized process of decarbonization without any specific timeframe. The Net Zero Emission scenario can assist in decarbonization of the major portion of polluting industries, however it will not facilitate the full decarbonization of industries that are not electrification-friendly. The Net Zero Emission scenario also includes widespread usage of sustainable fuels (biofuels, e-fuels, etc.) which many have already reached technical maturity but are limited in their adoption due to cost, environmental, and land use considerations. The Net Zero Emission scenario also includes a host of efficiency and material saving such as higher recycling rates and the usage of less cement in concrete production.

Many researchers have proposed the rapid expansion of hydrogen production and transmission infrastructure alongside the standard pathway of electrification and biofuel usage as a way of further decarbonizing the global economy 2. This adoption, often referred to as the "hydrogen economy", is what we will refer to as the "hydrogen pathway" of decarbonization. The hydrogen pathway envisions a maximum penetration of hydrogen into the global economy, and offers enormous potential which could completely change the landscape of decarbonization and the way that humans interact with energy. The hydrogen pathways also assists in providing important feedstock for hard-to-abate sectors, such as in the chemicals industry. However, the hydrogen pathway faces enormous challenges as seen for any energy transition, not to mention the fact that it has to overcome the global monopoly of fossil fuels as the primary source of portable, energy dense fuels. Hydrogen can also facilitate mass adoption of e-fuels which can operate in a closed loop of carbon cycling, but it will still require a constant level of CO₂ removal via direct air capture to maintain the closed loop.

Assumptions for calculating remaining emissions

• For ease of calculation, it is assumed that there is a parity relation to fuel usage and emissions. This means that a 30% reduction in fossil fuel usage will result in a 30% reduction in CO₂ emissions.
• It is also assumed that there is a parity relationship between the amount of CO₂ removed and the amount of required for synthesizing e-fuels. In other words, if fossil fuel usage accounts for 1Gt of emissions in a year and we want to replace it with e-fuels, we will need to remove 1Gt of CO₂ from the atmosphere in order to synthesize the e-fuels. This is not explicitly correct as the mass of CO₂ is different from the mass of pure carbon which is required for the synthesis of e-fuels, but it is a simplification that will allow rough estimates to be made.

Heavy-duty trucking

Heavy-duty trucking and freight transport are the sectors most primed for complete decarbonization through electrification, currently considered at the turning point of this transition ³. Although the pace is still slow, with only an estimated 1.2% of global truck sales in 2022 being electric, the rate at which this number is growing is accelerating rapidly ³.

The main challenges facing the electrification of heavy-duty trucks are the high capital costs of electric trucks and the limited availability of charging infrastructure and range issues. However, upfront costs are rapidly falling and many governments are already implementing national electrification plans ³.

The IEA's Net Zero Emission scenario estimates that heavy-duty trucking will reach 50% electrification along with 30% usage of sustainable fuels, reducing the annual emissions down from 1.8 Gt CO₂ to between 0.2 - 0.45 Gt CO₂ ^{1 2 4}.

Under the hydrogen pathway, much of the remaining emissions can be accounted for by the usage of e-fuels which combine green hydrogen with CO₂ or CO from direct air capture. This creates a closed loop of carbon cycling, but it will still require a constant level of CO₂ removal to maintain the closed loop, meaning that the remaining emissions will be the same as the Net Zero Emission scenario at 0.2 - 0.45 Gt CO₂.

Shipping

Shipping is a sector that will face higher degrees of difficulty in decarbonization compared to sectors such as heavy-duty trucking. Electrification of shipping is in its infancy stage with only two existing container ships capable of traveling up to 1000 km in operation in 2023 ³. Shipping requires a large amount of high-density energy to facilitate long-distance travel which currently cannot be provided by current battery technology.

Biofuels show the highest promise for decarbonization of shipping as the energy requirement is not so high that land usage considerations make it completely prohibitive, with some estimates putting the global potential (using fast-growing crops and agricultural residue without harming global food systems) of biofuels (114 Exajoules) to be approximately 11 times the total energy demand for shipping ^{3 5}. This can be further assisted by the fact that electrification of road-going vehicles could free up more supply of biofuels for shipping. As of 2024, current biofuel production sits close to 4.5 EJ so rapid but also sustainable scaling up would be required to meet the demand ³.

In the context of a hydrogen economy, e-fuels that have higher energy densities that are produced from hydrogen feedstock such as e-methanol (CH₃OH) and e-methane (CH₄), however these will all require some source of carbon feedstock for their synthesis. E-ammonia (NH₃) which is also sourced from hydrogen feedstock is another potential candidate, however ammonia has the disadvantage of being toxic, raising other issues ³. That withstanding, as the majority of shipping occurs in the oceans where there is almost no human population, the toxicity of ammonia in the seas is generally treated as less of a concern. That is not to say that the toxicity of ammonia to the environment is not a concern, but that its risks need to be weighed against the benefits of ammonia as a fuel. This is similar to how traditional shipping fuel known as "bunker fuel" contained a high sulfur content of up to 3.5% which would be considered unacceptable for land-based transport which have a maximum allowed sulfur content of 0.0016% in states such as California ^{6 7}.

Under the Net Zero Emission scenario, shipping will be able to decarbonize through the use of sustainable fuels, but total decarbonization through biofuels may not be achievable due to land usage concerns and the global availability of biofuels. The IEA estimates a high adoption of ammonia with 45% penetration, in addition to a 20% uptake of sustainable fuels ¹. Under this scenario, emissions can be reduced to 0.11 - 0.3 Gt CO₂ ^{1 2 4}.

Under the hydrogen pathway, the availability of ammonia will be vastly increased as it is readily sourced from green hydrogen. This could feasibly eliminate the remaining reliance on fossil fuels, effectively eliminating the remaining emissions from shipping, reducing the remaining emissions to 0.0 t CO₂.

Aviation

The aviation sector, like the shipping sector, is resistant to standard methods of electrification due to its strict requirement for high-density fuels in order to save weight. There has been interest in developing electric propulsion aviation, but it has mostly been blocked by the relatively low energy density of battery technologies. However, recent advances in batteries may make hybrid propulsion aircraft (turbine and battery) possible soon ³.

Similar to the shipping industry, biofuels appear to be the most likely option for decarbonization, as 'biojet' fuel can be synthesized and used directly in existing aircraft ³. Estimates of the global potential for sustainable biofuels sit around 8x the total energy requirement for aviation, but sustainable scaling up to meet this target will likely prove to be a complex task ^{3 5}.

Hydrogen produced from renewable sources could be utilized in the production of e-fuels such as e-kerosene in the same vein as the shipping industry. Due to hydrogen's lower volumetric energy density compared to other liquid fuels, it is considered unlikely that solely hydrogen-powered aircraft will be able to make an impact on decarbonization on long-distance flights³.

The IEA's Net Zero Emission scenario sees a small degree of electrification at 3% and the direct use of hydrogen at 8%, both for small aircraft on short-distance flights. Biojet makes up the majority of fuel usage at 45% while e-kerosene synthesized from green hydrogen and CO₂ accounts for 25% of fuel usage. The remaining 20% of fuel usage is made up of traditional fossil fuels ^{1 4}. Accounting for the fact that carbon dioxide needs to be removed from the atmosphere to maintain a closed loop of carbon cycling when producing e-kerosene, the remaining aviation emissions under the Net Zero Emission scenario are between 0.2 - 0.46 Gt CO₂ ^{1 2 4}.

For the hydrogen pathway, the remaining fossil fuel usage could be completely eliminated by the upscaled production of e-kerosene from green hydrogen and CO₂. However, to maintain the closed carbon loop, the requirement for direct air capture of CO₂ will remain the same as the Net Zero Emission scenario at 0.2 - 0.46 Gt CO₂.

Iron and Steel

The iron and steel industry represents the largest emissions segment of the hard-to-abate sectors, as well as being one of the most difficult to abate due to material processing requirements. Secondary steel production, where scrap steel is remelted using electric arc furnaces (EAF), does benefit from large scale electrification, with already approximately 22% of the world's total steel production being produced this way ³. Secondary steel production relies on a steady stream of scrap material to feed the furnace and the availability of scrap steel is predicted to almost double by 2050 in line with high usage over the past two decades ³. However, with the current rapid industrialization of emerging economies, around half of all steel production will still need to be newly created from primary production methods ³.

Primary production of steel traditionally relies on the cheap, effective, but highly polluting blast furnace reduction method utilizing metallurgical coke (high quality coal). Metallurgical coke is required to provide the high temperatures for the iron reduction process to occur, and more importantly, for providing carbon monoxide (CO) and dioxide (CO₂) required for the reaction to take place. The reduction process is essentially the separation of iron oxide (Fe₂O₃) to pure iron (Fe) and oxygen (O₂). Most if not all of the CO₂ is released into the atmosphere with approximately 1.4 tonnes of CO₂ released per tonne of steel created ³.

The technology that is expected to replace the traditional blast furnace reduction is called hydrogen-based direct reduced iron (DRI) which utilizes hydrogen instead of carbon monoxide for the reduction to take place. Hydrogen-based DRI requires large amounts of hydrogen, which is usually supplied by steam methane reforming (SMR) of natural gas (CH₄), a very emissions intensive process. Hydrogen-based DRI also requires a feedstock of high-quality iron ore with purities greater than 67% ⁸. As a comparison, the average iron ore purity found in China, the largest producer of iron ore in the world sits around 34%, and therefore could not be used directly for hydrogen DRI ⁸. High-quality iron ore that is suitable for hydrogen-based DRI is only found in a handful of locations, mainly in Australia and Brazil ⁸. If high-quality iron cannot be sourced, low-quality ore can be utilized after it has undergone the process of sintering (agglomeration), which essentially clumps the iron ore together to raise its purity. This sintering process requires temperatures greater than 1000°C and often still requires the presence of metallurgical coke to provide the conditions for reduction as well as a source of heat from other fossil fuel sources. It should be noted that the sintering process is also utilized to prepare the iron ore for the traditional blast furnace reduction process when iron ore quality is low or if the iron ore is in dust form ⁸.

For the net zero emissions pathway, it is estimated around 60% of all steel production can be sourced from scrap material and melted in electric blast furnaces, with scrap steel supply being the main bottleneck ^{8 1}. The remaining 40% will still require the use of a traditional blast furnace reduction process which will still emit CO₂. We can estimate the emissions using the following assumptions:

• 1.85 Gt of steel was produced in 2023 (Prod$_{2023}$), 72% from blast furnaces and 22% from scrap - electric arc furnaces ³
• In 2023, the emission intensity of blast furnace steel $(t_{BF})$ is between 1.82 - 2.45 t CO₂/t steel ³
• In the future scenario, the emission intensity of electric arc furnace steel $(t_\text{EAF})$ is between 0.4 - 0.6 t CO₂/t steel and it utilizes green electricity ³
• The emissions intensity of hydrogen-based DRI is assumed to be 0 t CO₂/t steel as it is produced from green hydrogen
• $(\%_{BF})$ and $(\%_{EAF})$ are the percentages of steel production that are still utilizing the blast furnace reduction process and the electric arc furnace process respectively.
• Assume that carbon capture and storage (CCS) is not utilized on the blast furnace steel production.

We can calculate the emissions intensity of steel production in 2023 as follows:

$$\begin{align*} \mathrm{Emissions_{\space \text{NZE}} = \text{Prod}_{2023} \times \text{\%}_{BF} \times t_\text{BF} + \text{Prod}_{2023} \times \text{\%}_{EAF} \times t_\text{EAF}} \end{align*}$$

Which results in an estimated CO₂ emission of between 1.79 - 2.48 Gt of CO₂ per year through the net zero emissions pathway where 60% of steel is secondary steel production and 40% is blast furnace steel production.

When considering the hydrogen pathway, we have to consider how much of the remaining primary steel production can be transitioned to renewable hydrogen-based DRI that produces no carbon emissions, with the main barrier being the availability of high-quality iron ore. We will assume that half of the remaining primary steel production can be transitioned to hydrogen-based DRI (60% scrap, 20% blast furnace steel, 20% hydrogen DRI), which will result in a reduction of emissions by 1.12 - 1.57 Gt of CO₂ per year, with these emissions accounting for the emissions resulting from iron ore preparing (sintering), as well as any remaining blast furnaces that will be necessary to continue meeting the growing demand for steel in emerging economies.

Chemicals and Petrochemicals

The chemicals and petrochemicals industry will be difficult to decarbonize under the standard pathway as much of the feedstock is derived directly from fossil fuels, or from hydrogen which is traditionally extracted from methane gas via steam methane reforming (SMR). However, more decarbonization is possible under the hydrogen pathway which can provide renewable hydrogen feedstocks and the ability to create synthetic fuels which can then be used to synthesize other important chemicals. For this analysis, we will focus on the production of ammonia, methanol, and high-value chemicals (HVCs) which account for 72% of the CO₂ emissions from the chemicals and petrochemicals industry ³.

Ammonia (NH₃)

Ammonia, a primary chemical used in nitrogen fertilizer, is unlikely to experience significant decarbonization under the standard pathway. In 2022, 72% of ammonia was sourced from natural gas while 26% was sourced from coal, contributing around 0.42 - 0.55 Gt of CO₂ emissions yearly ^{3 9}. Large scale electrification and process efficiency improvements will have little effect on reducing emissions from ammonia production as the main barrier lies with hydrogen sourcing.

Ammonia production would benefit greatly from the hydrogen pathway as it would allow for the production of ammonia from green hydrogen, circumventing the usage of natural gas and coal. Although the production of renewable ammonia will remain more expensive than conventional ammonia production for some time, a future hydrogen economy should allow for the production of ammonia from green hydrogen at a competitive price.

Under the net zero emissions pathway, 35% of ammonia production will be made using on site electrolyzers for industrial use, resulting in a remaining annual emissions of 0.273 - 0.358 Gt of CO₂ ¹.

Under the hydrogen pathway, it is well within the realm of reality that all ammonia production will be sourced from green hydrogen resulting in a remaining annual emissions of 0.0 t CO₂.

Methanol (CH₃OH)

Methanol production accounts for approximately 0.26 - 0.34 Gt of CO₂ emissions yearly and has the potential to be decarbonized under the standard pathway through the production of biomethanol, which can be produced via the anaerobic digestion of biomass ^{3 9}. However, this production runs into the same issues as any solution that relies on biofuels, as there is a practical limit to how much biofuel can be sustainably produced without harming global food systems, and the limited availability needs to be divided between other uses such as biojet fuel and biofuels for shipping. It should be noted that only 40% of the emissions related to methanol production comes from process emissions, while a little over half comes from feedstock production through the usage of steam methane reforming or coal reforming ¹⁰. As for the process emissions, the IEA estimates that 35% of energy demand will come from electricity, 10% hydrogen, 15% from bio-energy, with the remaining 40% coming accounted for by fossil fuels ¹.

Under the net zero emissions pathway, we can model a 60% reduction in energy usage in methanol production while maintaining the usage of steam methane reforming, which will result in a remaining annual emissions of 0.2 - 0.26 Gt of CO₂.

Under the hydrogen pathway, all feedstock emissions (50% of the original emissions) can be removed by the usage of green hydrogen by bypassing steam methane reforming, resulting in remaining emissions of 0.07-0.09 Gt of CO₂.

High-value chemicals (HVCs)

High-value chemicals used in the production of plastics, solvents, and pharmaceuticals are typically derived from fossil fuels and contribute around 0.25 - 0.33 Gt of CO₂ emissions yearly ^{2 9}. Decarbonization of HVCs is very similar to that of methanol, with some gains likely to come from biofuels and bioplastics; however, the extent to which these can provide sufficient feedstock is largely in question. Similarly, green hydrogen in conjunction with CO₂ sourced from direct air capture can be used to create the feedstocks necessary for the production of HVCs. There is a near 50:50 split of emissions stemming from energy usage and feedstock emissions ³. For energy improvements, the IEA estimates that 35% of energy demand will come from electricity for electronic steam cracking, while hydrogen (10%) and bio-energy (15%) will be used for conventional cracking, cracking being the process of breaking down large hydrocarbons into smaller ones necessary for the production of HVCs¹.

Under the net zero emissions scenario, we will assume that the majority of feedstock continues to be sourced from fossil fuels, while the process emissions are reduced by 60%, resulting in remaining annual emissions of 0.2 - 0.26 Gt of CO₂.

Under the hydrogen pathway, very little can change unless green hydrogen and direct air captured carbon are used to synthesize hydrocarbon feedstock. Even if this were the case, the annual emissions would remain the same (albeit in a closed carbon loop) at 0.2 - 0.26 Gt of CO₂.

Concrete (cement and lime)

Cement production is similar to the steel sector, as the challenge for decarbonization is twofold. On the one hand, the production of cement is very energy intensive, requiring high temperatures above 800°C, and on the other hand, the actual process of cement production releases CO₂ from the calcination of limestone ⁹. Cement slightly differs from steel in that there are other alternatives to cement usage, such as other building materials or methods that could be used instead of cement. However, the challenge arises from the fact that cement is highly cost-effective, ideal for construction, and it is so widely used that it is unlikely to be completely replaced by other materials. Because of this, it is important to find ways to decarbonize cement production without completely replacing it.

When talking about cement, we are almost always referring to Portland cement, which represents 95% of all cement production in the United States and relies on the calcination of limestone to produce the cement clinker, which is where the majority of CO₂ emissions are released ¹¹. Approximately 55% of all emissions related to cement production come from the calcination of limestone ¹². Alternatives to Portland cement exist, such as calcium sulfo-aluminate (CSA) cement, alkali-activated cement, and magnesium-based cements that all have lower CO₂ emissions, but it is estimated that these alternatives will only be able to make up 5% of all cement production by 2050 ¹³.

It should be noted however that the cement kilns often do not operate through the usage of fossil fuels, but rather the incineration of waste. In the US alone, around 70% of all hazardous waste is burnt in cement kilns, while in Europe, 50% of all recycled tires are burnt in cement kilns ¹³. The exhaust from this process is often heavily treated to remove hazardous materials such as sulfur dioxide and lead, but the CO₂ emissions are normally just released into the atmosphere.

The IEA's Net Zero Emissions scenario sees a 20% adoption of electrification of the calcination process through the usage of electric kilns and the potential usage of microwave technology which is still in the experimental stage ^{1 14 15}. This is combined with 30% usage of bio-energy and 5% hydrogen to fuel the cement kilns ¹. For process emissions in the calcination process and the combustion of fuel, the IEA relies heavily on the usage of CO₂ capture and storage (CCS) as the main form of reducing emissions; however, CCS is out of the scope of this analysis, and therefore we will assume that the process emissions remain unchanged. The aforementioned improvements in energy usage result in final remaining emissions of 1.74 - 2.03 Gt of CO₂.

Under the hydrogen pathway, it is possible that increased green hydrogen availability through the usage of onsite electrolyzers will allow for increased penetration of hydrogen-based cement kilns. For this analysis, we will assume that hydrogen usage will increase to 20% of total energy demand, resulting in remaining annual emissions of 1.56 - 1.82 Gt of CO₂.

Energy requirements for direct air capture

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 after decarbonization attempts have been made.

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:

• $\eta = 0.25$ for a 25% efficient capture process (table 3). Given the difficulties with capturing CO₂ in the low concentrations that it exists in air, achieving this would be considered a major breakthrough in DAC technology ¹⁶.
• $\eta = 0.078$ for a 7.8% efficient capture process, realistic efficiency of contemporary DAC systems (table 2) ¹⁷

Forecasting future energy requirements

In estimating the future emissions of hard-to-abate sectors under the Net Zero Emissions pathway and the hydrogen pathway, we assumed that the initial emissions were at the same level as in 2022. This ignores the fact that consumption and production of goods and resources are expected to continue to grow rapidly over time as more countries industrialize. Estimating future consumption is difficult as it depends on many factors, including the pace of industrialization, the development of new technologies, and the efficiency of resource utilization. It is reasonable to assume that the demand for materials and usage of transportation in the categories classified as hard-to-abate sectors will at least double over the time period considered, and some would argue that this is a conservative estimate ^{1 3 4}. Although a doubling of consumption does not necessarily mean that emissions will double, as modern processes tend to be less emissions-intensive than existing processes today, for the sake of simplicity, we have presented this doubling of consumption in all tables by the values in parentheses.

Table 3: Idealized capture energy for CO₂ from air assuming 25% thermal efficiency. Values in parenthesis represent a doubling of 2022 emissions levels

Sector	Net Zero Emissions Scenario [EJ]	NZE with Maximum Hydrogen Penetration [EJ]
Heavy-duty trucks	0.34 - 0.76 (0.68 - 1.51)	0.34 - 0.76 (0.68 - 1.51)
Shipping	0.18 - 0.50 (0.37 - 1.01)	0 (0)
Aviation	0.34 - 0.77 (0.68 - 1.55)	0.34 - 0.77 (0.68 - 1.55)
Iron and Steel	3.01 - 4.17 (6.02 - 8.34)	1.88 - 2.63 (3.76 - 5.26)
Chemicals and Petrochemicals	1.38 - 1.85 (2.76 - 3.70)	0.71 - 0.96 (1.41 - 1.92)
Ammonia (NH₃)	0.45 - 0.60 (0.91 - 1.20)	0 (0)
Methanol (CH₃OH)	0.34 - 0.43 (0.68 - 0.86)	0.12 - 0.15 (0.24 - 0.30)
High-value chemicals (HVCs)	0.34 - 0.43 (0.68 - 0.86)	0.34 - 0.43 (0.68 - 0.86)
Concrete (cement and lime)	2.92 - 3.41 (5.84 - 6.82)	2.62 - 3.06 (5.24 - 6.12)
Total	8.16 - 11.43 (16.33 - 22.92)	5.88 - 8.18 (11.76 - 16.36)

Material requirements for direct air capture

Previously we calculated the material requirements for the production of solid sorbent direct air capture systems, focusing mainly on the production of the sorbent. From this, it is possible to calculate the material requirements for the production of solid sorbents used to capture 1 kg of CO₂ from the atmosphere. Using these values, we can extrapolate these material requirements to estimate how much would be required for the annual abatement of the remaining emissions of the hard-to-abate sectors. The material consumption for he manufacture of amine polyethyleneimine (PEI), a commonly used solid sorbent material, for 1kg of CO₂ captured is shown in table 4, while the annual consumption for the capture of the total emissions from hard to abate sectors located in table 5.

Table 4: Life cycle inventory for the consumption of PEI sorbent per kg of CO₂ captured

Process Name	Input/Output	Amount (per kg CO₂)	Unit
Input
Aziridine	Ethanolamine	0.00594	kg
	Sulfuric acid	0.00954	kg
	Sodium hydroxide	0.00777	kg
Homopolymerization	Hydrochloric acid (35%)	0.00060	kg
	Sodium hydroxide	0.00060	kg
	Ethanol	0.01035	kg
	Diethyl ether	0.12363	kg
	Water	0.04368	kg
Energy	Electricity	0.00105	kWh
	Heat	0.02244	MJ
Output
	Sodium sulfate	0.01380	kg
Treatment	Unreacted raw materials/solvents	0.00450	kg

Table 4: Life cycle inventory for the consumption of PEI sorbent per kg of CO₂ captured

Process Name	Input/Output	Net Zero Emissions Scenario (NZE)	NZE with Maximum Hydrogen Penetration	Unit
Input
Aziridine	Ethanolamine	28.9 - 40.5 (57.7 - 81.0)	20.8 - 28.9 (41.6 - 57.8)	Mt
	Sulfuric acid	46.3 - 65.0 (92.7 - 130.0)	33.4 - 46.5 (66.8 - 92.9)	Mt
	Sodium hydroxide	37.8 - 53.0 (75.6 - 106.0)	27.2 - 37.9 (54.4 - 75.8)	Mt
Homopolymerization	Hydrochloric acid (35%)	2.92 - 4.09 (5.83 - 8.18)	2.10 - 2.92 (4.20 - 5.84)	Mt
	Sodium hydroxide	2.92 - 4.09 (5.83 - 8.18)	2.10 - 2.92 (4.20 - 5.84)	Mt
	Ethanol	50.3 - 70.6 (101.0 - 141.0)	36.3 - 50.5 (72.5 - 101.0)	Mt
	Diethyl ether	601.0 - 843.0 (1200.0 - 1690.0)	433.0 - 602.0 (866.0 - 1200.0)	Mt
	Water	212.0 - 298.0 (424.0 - 596.0)	153.0 - 213.0 (306.0 - 426.0)	Mt
Energy	Electricity	5.10 - 7.16 (10.2 - 14.3)	3.68 - 5.11 (7.35 - 10.2)	TWh
	Heat	0.109 - 0.153 (0.218 - 0.306)	0.0785 - 0.109 (0.157 - 0.218)	EJ
Output
	Sodium sulfate	67.1 - 94.1 (134.0 - 188.0)	48.3 - 67.2 (96.6 - 134.0)	Mt
Treatment	Unreacted raw materials/solvents	21.9 - 30.7 (43.8 - 61.4)	15.8 - 21.9 (31.6 - 43.8)	Mt

Sources

Footnotes

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3. International Renewable Energy Agency. (2024). Decarbonising hard-to-abate sectors with renewables: Perspectives for the G7. International Renewable Energy Agency. https://www.irena.org ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶ ↩¹⁷ ↩¹⁸ ↩¹⁹ ↩²⁰ ↩²¹ ↩²² ↩²³

4. Groppi, D., Pastore, L. M., Nastasi, B., Prina, M. G., Garcia, D. A., & de Santoli, L. (2025). Energy modelling challenges for the full decarbonisation of hard-to-abate sectors. Renewable and Sustainable Energy Reviews, 209, 115103. https://doi.org/10.1016/j.rser.2024.115103 ↩ ↩² ↩³ ↩⁴ ↩⁵

5. International Renewable Energy Agency. (2016). Boosting biofuels: Sustainable paths to greater energy security (ISBN: 978-92-95111-84-4). Retrieved from https://www.irena.org/publications/2016/Apr/Boosting-Biofuels-Sustainable-Paths-to-Greater-Energy-Security ↩ ↩²

6. Yuan, T., Song, H., Oreopoulos, L., & others. (2024). Abrupt reduction in shipping emission as an inadvertent geoengineering termination shock produces substantial radiative warming. Communications Earth & Environment, 5, Article 281. https://doi.org/10.1038/s43247-024-01442-3 ↩

7. California Air Resources Board. (2020). Rule 431: Sulfur content of fuels. (Originally adopted in 1976). Retrieved from https://ww2.arb.ca.gov/sites/default/files/classic/technology-clearinghouse/rules/RuleID4734.pdf ↩

8. Wei, C., Zhang, X., Zhang, J., Xu, L., Li, G., & Jiang, T. (2024). Development of direct reduced iron in China: challenges and pathways. Engineering. https://doi.org/10.1016/j.eng.2024.04.025 ↩ ↩² ↩³ ↩⁴ ↩⁵

9. 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

10. Hamelinck, C., & Bunse, M. (2022). Carbon footprint of methanol. Gear Up. https://www.methanol.org/wp-content/uploads/2022/01/Carbon-Footprint-of-Methanol-studio-Gear-Up-Full-Presentation.pdf ↩

11. U.S. Environmental Protection Agency. (2025). AP 42, Section 11.6: Portland cement manufacturing. Retrieved from https://www.epa.gov/air-emissions-factors-and-quantification/ap-42-compilation-air-emission-factors ↩

12. Busch, P., Kendall, A., Murphy, C. W., & Miller, S. A. (2022). Literature review on policies to mitigate GHG emissions for cement and concrete. Resources, Conservation and Recycling, 182, 106278. https://doi.org/10.1016/j.resconrec.2022.106278 ↩

13. Habert, G., Miller, S. A., John, V. M., Provis, J. L., Favier, A., Horvath, A., & Scrivener, K. L. (2020). Environmental impacts and decarbonization strategies in the cement and concrete industries. Nature Reviews Earth & Environment, 1(9), 559-573. https://doi.org/10.1038/s43017-020-0093-3 ↩ ↩²

14. Xiao, Y., & Xue, Y. (2024). A review on application of microwave in cement life cycle. Renewable and Sustainable Energy Reviews, 199, 114498. https://doi.org/10.1016/j.rser.2024.114498 ↩

15. Vermeiren, J., Dilissen, N., Goovaerts, V., & Vleugels, J. (2024). Electrification of clinker and calcination treatments in the cement sector by microwave technology – A review. Construction and Building Materials, 428, 136271. https://doi.org/10.1016/j.conbuildmat.2024.136271 ↩

16. House, K. Z., Baclig, A. C., Ranjan, M., van Nierop, E. A., Wilcox, J., & Herzog, H. J. (2011). Economic and energetic analysis of capturing CO₂ from ambient air. Proceedings of the National Academy of Sciences of the United States of America, 108(51), 20428–20433. https://doi.org/10.1073/pnas.1012253108 ↩

17. Long-Innes, R., & Struchtrup, H. (2022). Thermodynamic loss analysis of a liquid-sorbent direct air carbon capture plant. Cell Reports Physical Science, 3(3), Article 100791. https://doi.org/10.1016/j.xcrp.2022.100791 ↩