The environmental impact of producing solid sorbent materials for direct air capture is significant, requiring hundreds of millions of kilograms of feed stock depending on what climate targets are being achieved

Summary

Life Cycle Assessment reveals that while DAC plant construction has minimal environmental impact, solid sorbent production and disposal present significant material challenges. Using polyethyleneimine (PEI) sorbents as an example, current consumption rates of 7.5g sorbent per kg CO₂ captured (3g PEI at 40% loading) would require massive material inputs at climate-relevant scales. For 1300 Gt CO₂ capture by 2100 which is required to achieve the goals of the Paris agreement, production would demand 12.4 trillion tons of sulfuric acid and 56.8 trillion tons of water - orders of magnitude beyond current US industrial production levels of sulfuric acid, but an order of magnitude less than the amount of water consumed by thermal power plants in the United States in 2021. These estimates highlight the need for improved sorbent durability and recycling to reduce material intensity.

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In order to understand the environmental impacts of a solid sorbent direct air capture (DAC) system, we can utilize the industry standard methodology known as Life Cycle Assessment (LCA). Life cycle assessment is a set of methods that allows researchers to quantify all material inputs and waste outputs of a given process or piece of technology from cradle-to-grave. Cradle-to-grave refers to the entire life of a product or process, from the extraction of raw materials to the disposal of waste products.

As previously discussed, the actual construction of a solid sorbent system is considered a relatively simple process, mainly consisting of a fan to pass air through an air contactor box which contains the solid sorbent material. Life cycle assessments using data sourced from the only commercially operational solid sorbent DAC plants operated by Climeworks show that the environmental impacts of the initial construction of DAC plants are negligible ¹. The novel environmental impact arising from material inputs comes from the production and disposal of the solid sorbent materials that are contained within the air contactor box. Solid sorbents have the difficult task of selectively capturing CO₂ from air in concentrations around 420 parts per million, quite literally like finding a needle in a haystack. Because of this, some of the potential materials used as solid sorbents are highly reactive so that they can capture any CO₂ molecule that passes close by.

The outputs of a typical life cycle analysis present the results in terms of environmental impact categories which use fairly abstract methods of quantifying the environmental impact of a given process, such as: human toxicity (cancer), particulate matter, ozone depletion and ionizing radiation. These categories are incredibly useful in quantifying the environmental impact of a given process, but they are not always useful in communicating the actual environmental impact of the process. Another way of evaluating the environmental impact of the sorbent production process is to quantify the total mass of material inputs and waste outputs which can be easier to visualize.

For this, we will focus on a typical solid sorbent amine 'Polyethyleneimine' (PEI) which is a commonly used solid sorbent material in the DAC industry ^{1 2}. PEI is a synthetic polymer that is highly suited for capturing CO₂ from air as it has a high amine content, meaning that it can bind to CO₂ molecules, and it is a cationic (positively charged) molecule, making it attractive to CO₂ molecules which are negatively charged.

Sorbent Production

Note: The following analysis is rudimentary and should be interpreted with high degree of caution. As the technology is still in its infancy, there is a noted lack of data relating to inventories available for researchers to analyze and many of the processes are considered trade secrets ¹. As the technology develops, more open data will be available to make more accurate and detailed analyses.

Assumptions:

1. The amine loading (weight percentage of amine to amine support) is 40% by weight. Depending on the sorbent support material (e.g. silica, alumina, etc.) the amine loading can vary from around 30% – 70% ².

2. For every 1kg of CO₂ captured, 7.5g of the sorbent (amine and support material) is consumed. This value is derived from experimental data and information from Climeworks ¹. It is also estimated that future solid sorbent DAC plants will consume around 3g of PEI per kg of CO₂ captured.

Table 1: Life cycle inventory for the production of 1kg of PEI sorbent

Process Name	Input/Output	Amount (avg. best/worst case) 1	Unit
Input
Aziridine	Ethanolamine	1.98	kg
	Sulfuric acid	3.18	kg
	Sodium hydroxide	2.59	kg
Homopolymerization	Hydrochloric acid (35%)	0.20	kg
	Sodium hydroxide	0.20	kg
	Ethanol	3.45	kg
	Diethyl ether	41.21	kg
	Water	14.56	kg
Energy	Electricity	0.35	kWh
	Heat	7.48	MJ
Output
	Sodium sulfate	4.60	kg
Treatment	Unreacted raw materials/solvents	1.50	kg

Using the assumed amine loading of 40% by weight, we can calculate the mass of PEI consumed per kg of CO₂ captured:

$$\begin{align*} \text{PEI}_{consumed} &= 7.5 \text{g} \times 40\%\\ &= 3 \text{g} \end{align*}$$

Given that 3g of PEI is consumed per kg of CO₂ captured, we can calculate the quantity of inputs and outputs of PRI production per kg of CO₂ captured:

Table 2: Life cycle inventory for the consumption of PEI sorbent per kg of CO₂ captured

Process Name	Input/Output	Amount	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

We can then extrapolate this data for a given desired mass of CO₂ captured. Using the values derived from the International Panel on Climate Change (IPCC) Representative Concentration Pathways (RCPs), we know that around 1300 Gt of CO₂ needs to be captured by 2100 to achieve the goals of the Paris agreement (RCP2.6). To do this, we multiply each value by 1.3 × 10¹⁵ kg.

Table 3: Life cycle inventory for the production of PEI sorbent for the capture of 1300 Gt of CO₂

Process Name	Input/Output	Amount	Unit
Input
Aziridine	Ethanolamine	7.722 × 10¹²	kg
	Sulfuric acid	1.2402 × 10¹³	kg
	Sodium hydroxide	1.0101 × 10¹³	kg
Homopolymerization	Hydrochloric acid (35%)	7.8 × 10¹¹	kg
	Sodium hydroxide	7.8 × 10¹¹	kg
	Ethanol	1.3455 × 10¹³	kg
	Diethyl ether	1.60719 × 10¹⁴	kg
	Water	5.6784 × 10¹³	kg
Energy	Electricity	1.365 × 10¹²	kWh
	Heat	2.9172 × 10¹³	MJ
Output
	Sodium sulfate	1.794 × 10¹³	kg
Treatment	Unreacted raw materials/solvents	5.85 × 10¹²	kg

To put these values into perspective, the total production of sulfuric acid (H₂SO₄) in the United States in 2019 was 22,845 million kg ³, compared to the 10,101,000 million kg of sulfuric acid that would be consumed in the production of PEI sorbent for the capture of 1300 Gt of CO₂. Regarding water usage, in 2021, the total water consumption by thermal power plants (which account for approximately 49% of all water usage in the United States) was 47.7 trillion gallons, or 1.81 × 10¹⁴ liters - an order of magnitude higher than the estimated 5.6784 × 10¹³ liters of water that would be used in the production of PEI sorbent for the capture of 1300 Gt of CO₂ ^{4 5}.

Sources

Footnotes

1. Deutz, S., & Bardow, A. (2021). Life-cycle assessment of an industrial direct air capture process based on temperature–vacuum swing adsorption. Nature Energy, 6(2), 203-213. https://doi.org/10.1038/s41560-020-00771-9 ↩ ↩² ↩³ ↩⁴ ↩⁵

2. Panda, D., Kulkarni, V., & Singh, S. K. (2023). Evaluation of amine-based solid adsorbents for direct air capture: A critical review. Reaction Chemistry & Engineering, 8(1), 10-40. https://doi.org/10.1039/D2RE00211F ↩ ↩²

3. U.S. Environmental Protection Agency. (2023). Sulfuric acid supply chain profile. https://www.epa.gov/system/files/documents/2023-03/Sulfuric%20Acid%20Supply%20Chain%20Profile.pdf ↩

4. U.S. Environmental Protection Agency. (n.d.). Lean water toolkit: Chapter 2. U.S. Environmental Protection Agency. https://www.epa.gov/sustainability/lean-water-toolkit-chapter-2 ↩

5. U.S. Energy Information Administration. (2022, October 31). Nearly half of U.S. water withdrawals are for thermoelectric power generation. Today in Energy. https://www.eia.gov/todayinenergy/detail.php?id=56820 ↩