Cement Production: The Role of Clinker and CO₂ Emissions
To understand the potential impact of integrating the FlashPhos process into a cement plant, it is first necessary to have a basic understanding of cement production. Cement is the hydraulic binder used in concrete. It is produced from so-called cement clinker, which is formed in a high-temperature process known as clinker burning, using a raw material mixture of limestone and clay.
An important step in this process is the calcination of the limestone in the preheater system and precalciner, where calcium carbonate (CaCO₃) is decomposed at around 900°C into calcium oxide (CaO) and carbon dioxide (CO₂). This is followed by the sintering of the decarbonized material in the rotary kiln at around 1450°C. During this stage, the necessary clinker phases are formed from calcium oxide (CaO), silicon (Si), and other components. The resulting clinker granules are then ground together with cement additives to produce the cement powder as it is commonly known.
Two-thirds of the CO₂ emissions in a cement plant are attributable to the calcination process and are considered unavoidable. One-third of the CO₂ emissions results from the thermal energy required for the process, which is generated by burning fuels. These emissions can theoretically be avoided through fuel substitution with renewable energy sources.
By using sustainable pyrolysis gases from the FlashPhos process in a cement plant, synergy effects may arise, allowing for savings in primary energy and thus reductions in CO₂ emissions. In addition, the slags from the FlashPhos process also show potential as a secondary and CO₂-free cement material.
A Sector in Transformation
In response to mounting environmental concerns, the cement and concrete industry is actively pursuing a roadmap toward CO₂ minimization. This involves several strategic shifts:
- Reducing direct CO₂ emissions from clinker production.
- Replacing fossil fuels with alternative energy sources.
- Rethinking the composition of cement and concrete products.
- Exploring the use of alternative raw materials that can reduce the carbon footprint without compromising quality.
Alternative Fuels and Materials: A Mixed Bag
One way to reduce emissions is the use of alternative fuels in cement kilns. In 2021, the cement industry processed around 665,000 tons of sewage sludge. However, because sewage sludge contains significant amounts of phosphorus, its incineration in European cement plants will no longer be permitted starting in 2028.
The FlashPhos process offers a solution to continue utilizing the energy content of sewage sludge in cement kilns in the future. Through the FlashPhos process, the phosphorus contained in the sewage sludge is separated and recovered as a product, and part of the energy can be made available to the cement process in the form of pyrolysis gas. This could theoretically reduce fuel-related CO₂ emissions.
Another synergy effect may come from the use of FlashPhos slags as an alternative cement material, which could also lead to savings in raw material-related CO₂ emissions.
FlashPhos: A Circular Breakthrough
The FlashPhos process represents a promising innovation. It is a thermochemical process developed for the recovery of high-quality phosphorus from sewage sludge. The integration of this process into a cement plant or another host plant is based on the idea that process synergies can be harnessed to make both processes more efficient in terms of material use and energy.
The FlashPhos process aims to address several challenges simultaneously:
- Phosphorus is no longer co-incinerated but recovered and can be used for other purposes.
- Slags with hydraulic binding properties are produced, which can partially replace cement clinker.
Ideally, energy utilization can be increased and CO₂ emissions reduced.
The Hypotheses are:
- FlashPhos slags can be used as secondary cementitious material (SCM) in cement production
- Hot pyrolysis gases as outputs from the FlashPhos process can be used as fuel gas input for the cement plant
- Utilization of low temperature excess heat from the cement plant to dry sewage sludge
First analyses show:
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Grindability and cementitious properties of artificial FlashPhos slags are in the lower range of GGBFS performance
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Feeding FlashPhos fuel gases into the calciner is evaluated as the best possibility Using FlashPhos slags as SCM, requires REACH registration at the European Chemical Agency. This is very time-consuming and expensive.
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The use of slag as an alternative and CO2-free raw material is not subject to any strict conditions.
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It was decided that sewage sludge drying can only be decentralized, therefore no excess heat from the cement plant can be used for SS drying
Toward Systemic Extended Value Chains
FlashPhos exemplifies a shift toward what we call systemic extended value chains. Rather than treating waste as an endpoint, this approach reimagines waste streams—like sewage sludge—as sources of value that can feed into other industrial sectors. In doing so, it creates circular loops that benefit both the environment and the economy.
For the cement and concrete industry, this means access to alternative raw materials that reduce reliance on virgin resources and lower CO₂ emissions. Early results, such as the activity index of cements incorporating FlashPhos slag, suggest strong potential for maintaining performance while enhancing sustainability.
Conclusion
The path to climate-neutral cement is complex and requires systemic change across the entire production chain. By integrating phosphorus recovery with cement production, FlashPhos is helping to turn one industry’s waste into another’s solution. It’s a clear example of how innovation and cross-sector collaboration can unlock new value in the transition to a circular, low-carbon economy. However, process optimization and more research are needed in this field.