Lime kilns are everywhere, from cement and steelmaking to agriculture and paper production, and they’re responsible for nearly 8% of global CO₂ emissions. What would it mean for the world if this ancient technology could not only stop emitting, but actually start contributing to the removal of carbon dioxide from the atmosphere?
Thinking Across Sectors
A key step in tackling climate change is understanding where emissions come from. When we map global greenhouse gas emissions by sector, the picture usually looks something like this (source: Visual Capitalist):
Sector | tCO2e (2022) | % of Total |
Electricity & Heat | 16.7B | 33% |
Transportation | 8.2B | 16% |
Manufacturing & Construction | 6.3B | 13% |
Residential & Commercial | 3.2B | 6% |
Fugitive Emissions | 31.B | 6% |
Military Fuel Use | 603M | 1% |
Agriculture | 5.9B | 12% |
Land-Use Change & Forestry | 1.3B | 3% |
Industrial Processes | 3.2B | 6% |
Waste | 1.7B | 3% |
Total | 50.1B | N/A |
But looking at emissions by sector can obscure a deeper truth.
When we divide emissions between industries, what we’re implicitly asking is: “The operations of which industries do we need to reduce?” But as we’ve explained in our post on Value Chain Decarbonization (VCD), if we change key technologies within an industry, the overall emissions of that industry or sector’s operations can actually turn negative, and start contributing to environmental rehabilitation; in cases like these the expansion of operations can in fact be desirable. Not only that, but some sectors share technologies and practices, which can be modified for environmental rehabilitation.
By framing the problem as solely sector-oriented, we run the risk of overlooking opportunities for decarbonization across sectors and technologies, as well as of missing chances to collaborate with and harness industries for environmental causes. By drilling down and finding “big problem” technologies, rather than sectors, we can uncover new pathways for carbon debt reduction across the entire economy.
Lime: A Hidden Cornerstone of Industry
Lime-producing technology is one of the most widely utilized industrial processes on Earth. In addition to processes and industries in which lime is used as a feedstock, like steelmaking, construction, agriculture, pulp and paper, and sugar refining, among many more, lime kilns are also used in an essential step in cement-making, though the lime they produce is only an intermediary substance in the process. Unfortunately, lime production is also inherently, and extremely, carbon intensive.
Cement alone accounts for roughly 8% of global emissions, most of it due to in-house lime production (World Economic Forum, 2024).
Producing lime is usually done by heating up limestone (CaCO₃) until it breaks down into lime (CaO) and CO₂. This process generates two types of emissions:
- Process emissions: CO₂ released from the limestone itself.
- Combustion emissions: CO₂ produced by burning fuels to reach temperatures of 900–1,000°C.
On average, 60–70% of lime’s emissions come from the chemical process itself, and 30%–40% from fuel combustion (EuLA Innovation Booklet, 2023).
From a cross-sector perspective, this makes lime kilns a prime candidate for decarbonization. The technology itself, not the industries or sectors that use it, is the problem and – due to lime’s excellent CO2 binding properties, as we’ll explain in below – also the opportunity.
Why Lime Is So Hard to Decarbonize
Lime kilns operate at some of the highest temperatures in industry (around 1,000°C) to fully decompose limestone. Not only does reaching this heat require immense amounts of energy – keeping lime kilns at these operational temperatures is actually more energy-demanding than you might think. The decomposition of limestone into lime is an endothermic process; in chemical terms, that means it’s a process which absorbs heat, rather than releases it. That means that in order to maintain the required 1,000°C, more and more heat needs to be continually added – a substantial engineering problem that is currently solved by fuel combustion. The carbon dioxide emitted by both the limestone and the burning of fuels is complicated to capture and handle.
As a result, for every kilogram of lime produced, between 1 and 1.4 kilograms of CO₂ are emitted, depending on the hydration level of the product. Given lime’s ubiquity, the cumulative impact is staggering – and seemingly inescapable.
The Carbon Removal Potential of Lime
Perhaps ironically, the same chemistry that makes lime such a major emitter also makes it a natural CO₂ sponge. Lime “loves” bonding with carbon dioxide – a process called carbonation. Simply exposed to air, lime will gradually absorb 0.5-0.8 kg of CO₂ per kg of material, depending on the exact use it’s been put to (Literature Review on the Assessment of the Carbonation Potential of Lime in Different Markets and Beyond). When used in applications like cement or steel, it typically reabsorbs about 0.4 kg of CO₂ per kg of lime. It’s so effective at absorbing CO2, that once bonded, it requires huge amounts of energy to separate the two – which explains why lime production methods are so energy-intensive.
Today, the CO₂ emitted in lime production far outweighs what the material later reabsorbs. But imagine if we could produce lime while capturing its emissions at the source, or better – without emitting them in the first place.
In such a world, lime would transform from a massive emitter to a carbon dioxide remover.
The avoided emissions from decarbonized production would erase around 8% of global CO₂ emissions, while the lime itself would go on to capture additional CO₂ from the atmosphere as it’s used, and as a natural “side effect” of existing, essential industrial processes.
Lime would become a cornerstone not only of industry, but of carbon dioxide removal (CDR) and environmental rehabilitation.
To learn how CarbonBlue’s EcoLime technology is making this transformation a reality, click here.
