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Figure 1. GCCA’s net-zero parthway. Source: Global Cement and Concrete Association’s Cement Industry Net Zero Progress Report.

The industry has already been moving toward decarbonization for several years, employing various technologies. However, with numerous options available, plant environmental teams often feel overwhelmed, lacking clear information on the most cost-effective methods to reduce CO₂ emissions for their specific circumstances. This article focuses on decarbonizing clinker and cement production, exploring different possibilities and proposing scenarios tailored to each plant’s stage on the decarbonization path.

Pathways for a Sustainable Future

CO₂ emissions in clinker and cement production can be divided into two primary sources: emissions from raw material decarbonization and emissions from combustion. Several strategies exist to mitigate both, discussed in detail below. To facilitate comparison, we’ll use a hypothetical “standard kiln” with 4,000 tpd capacity, 800 kcal/kg thermal consumption, and a 40%-60% energy split between the kiln and calciner respectively, operating for 8.000 hours per year.

• Increase plant efficiency / improve clinker quality

Improving plant efficiency has a very broad meaning and has influence in all other methods presented as well. CO2 emissions are directly related to the amount of energy, thermal or electric, to produce 1 kg of material. Reducing the energy required to produce 1 kg of material lowers fuel consumption and the volume of gases generated, thereby reducing both thermal and electrical energy needs.

For instance, in the above-mentioned standard kiln, a 3% efficiency improvement would reduce CO₂emissions by approximately 14.000 tons annually.

It might not seem much but considering that a 3% improve in the plant efficiency can be achieved by simple actions and low investment, involving better process control, reducing false air in the system, optimizing raw meal composition to allow for easier cooking, upgrading to a combustion system that allows the operation with lower oxygen content in the kiln while keeping the combustion under control are some examples.

Similarly, enhancing clinker quality can lead to significant CO₂ reductions through the whole process. A better-quality clinker means that less clinker can be used, to achieve the same cement strength. A 3% reduction in the clinker factor due to better clinker quality in our standard plant scenario would decrease CO₂ emissions by around 34.000 tons per year. Combining this with a 3% efficiency improvement results in a total reduction of approximately 47.000 tons annually.

Figure 2. A high-quality clinker can reduce the clinker factor in cement, therefore reducing the CO₂emissions. Source: public domain

Methods to improve clinker quality include better process control, raw meal composition optimization and homogeneity, and investing in advanced combustion systems such as the Turbu-Flex™ from FCT.

These options are often overlooked powerful alternatives, at relatively low costs, to allow plants to contribute to their initial share of CO₂ emissions reduction.

• Use of alternative fuels

It might sound counterintuitive that the use of alternative fuels would reduce CO₂ emissions, as their combustion also releases CO₂. While the combustion of alternative fuels also releases CO₂, it prevents additional fossil fuel consumption and their associated emissions. Alternative fuels, often by-products of other industries, can replace fossil fuels, reducing overall carbon emissions. In this way, the carbon from the alternative fuels is once more reused after their primary purposes, instead of using a brand-new font of carbon taken from fossil fuels.

The use of alternative fuels is a mature technology, with plants in Europe being able to achieve up to 90% substitution rates by solid alternative fuels, and up to 100% when considering a mix of solid and liquid alternative fuels.

The cost of converting a plant to high substitution rates varies based on factors such as plant design, existing equipment residence times, the quality of alternative fuels that can be sourced /prepared, technology of the combustion equipment among others.

For our standard kiln, a 10% increase in thermal substitution rate replacing coal with RDF could reduce CO₂ emissions by around 6.000 tons per year. Achieving a 90% substitution rate would reduce emissions by approximately 51.000 tons per year.

Figure 3. State-of-the-art Turbu-Flex^TM burner from FCT for high substitution rates by alternative fuel. Source: FCT

Investments in CFD studies for the calciner and advanced combustion systems like the Turbu-Flex™ kiln burner from FCT are relatively low investments that can facilitate this transition.

• Replacement of clinker per supplementary cementitious materials

Materials that possesses binding characteristics when mixed with water or other materials, known as supplementary cementitious materials (SCMs), have been used for several decades as a clinker replacement. Fly ash from power plants running on coal, slags from steel production and natural pozzolanic material are common examples of SCMs. However, their availability is decreasing, prompting the search for alternatives, with calcined clay being a prominent option which is used in cement for over 40 years in some countries like Brazil for instance.

The clay used for the calcination is an aluminosilicate mineral within certain ratios of alumina and silica, containing hydroxyl in its structure. Once the clay is submitted to thermal treatment at the right temperatures, the hydroxyl is released from the clay structure. When the clay is mixed with cement, the Calcium oxide from the clinker will react with the clay, giving its strength. The right temperature for clay treatment is very particular of each clay, but in any case, it is below the clinkerization temperatures. Therefore, two benefits in terms of CO₂ reduction can be achieved when replacing clinker by clay in the cement: the CO₂ emissions from the combustion are reduced (there are further benefits if alternative fuels are used), and there are virtually no CO₂ emissions from the material (except for some organic and limestone traces).

Once more, the investment costs of this possibility are within a very wide range. On one side, existing clinker kilns can be adapted for clay calcination with minor adjustments, offering a cost-effective solution, although not the most efficient, even with a pendular operation between clinker and clay. On the other side of the scale, new plants using rotary kilns or flash calciner technologies can be built for optimal efficiency, with an array of several different configurations. FCT offers the expertise of our engineers with more than 40 years in clay calcination and technologies like the RotaCalx™ (rotary kiln) and FlashCalx™ (flash calciner) for clay production.

Figure 4. dried and ground raw clay (top left), calcined clay without color control techniques (top right) and calcined clay with FCT’s patented color control process (bottom). Source: FCT

For our standard kiln, replacing 40% of clinker with calcined clay could reduce CO₂ emissions by around 347.000 tons yearly.

• Use of hydrogen

Considering that hydrogen gas is a fuel that has no carbon, it should be considered ideal to reduce CO₂emissions. So far, the use of hydrogen in cement production has been limited to very small amounts, in the range of single digits of percentage, from 0,2 to 3%. The limitation is rather in the production than in the combustion, especially when considering an environmentally friendly production of hydrogen. Nowadays, the vast majority, above 99%, of the hydrogen produced, is coming from the reform of natural gas which releases a substantial amount of CO₂ to the atmosphere. If this is the case, burning natural gas directly in the kiln has a lower environmental impact. Apart from that, hydrogen is still expensive vis-à-vis other fuels and is not largely available to the industry hence it is not a feasible option at the moment for cement production.

In case environmentally friendly hydrogen, for example produced via electrolysis of water using solar or wind power, becomes available at affordable prices in the future, there are some challenges to use the hydrogen in cement production in large scale, including reduced flame emissivity, increased NOx emissions and distribution infrastructure for instance. However, these hurdles are effectively significant only when achieving a substitution rate by hydrogen above 50%. There are important investments in order to make hydrogen production more affordable, in larger quantities and clean. FCT wrote a comprehensive article in World Cement in November 2022 discussing the aspects around hydrogen usage in clinker kilns.

FCT’s burners are currently delivered for hydrogen use, ready for when clean and affordable hydrogen becomes available.

For our standard kiln, replacing 50% of coal with hydrogen would reduce CO₂ emissions by around 29.000 tons per year.

• Oxyfuel process

Oxyfuel combustion is an innovative technology that can play a crucial role in reducing CO₂ emissions in the cement industry. In a traditional cement kiln, fuel is burned in air, which contains about 21% oxygen and 78% nitrogen in volume. The nitrogen in the air does not contribute to the combustion process and instead ends up in the flue gas, diluting the CO₂ concentration. Oxyfuel combustion, on the other hand, involves burning fuel with pure oxygen instead of air, recirculating part of the CO₂ from the exhaust gases back to the cooler. This method results in a more efficient and intense combustion process and a significant reduction in nitrogen oxide (NOx) emissions. Oxyfuel combustion does not reduce the CO₂ emissions per se, but it produces a flue gas that is rich in CO₂, making it easier and more cost effective to be captured and stored or utilized.

This technology is new and has its challenges that need to be addressed, such as pure oxygen availability and costs, retrofit of existing plants as well as adaptations of the operational conditions.

Figure 6. Combustion represented by an isotherm surface coloured by CO molar fraction, on top: oxyfuel combustion, at the bottom: conventional combustion with air. Source: FCT

Considering the standard kiln in this article, implementing oxyfuel combustion could lead to a significant reduction in CO₂ emissions. With the ability to capture a high percentage of CO₂ from the flue gas using a CCUS (carbon capture, utilization and storage) system, oxyfuel combustion could potentially allow the easier reduction of CO₂ emissions by up to around 900.000 ton per year, depending on the specific conditions and capture efficiency.

FCT burners are designed to operate under oxyfuel combustion conditions with minor adjustments.

• Green electrification, Carbon Capture, Use and Storage (CCUS) and Others

There are few initiatives in the field of direct electrification and use of thermal carriers to replace the combustion in clinkerization processes. However, they are still very incipient and are further away in time when compared to the alternatives discussed here. The main challenges are to build reliable equipment and materials that can “deliver” 1450°C to the clinker, a reliable source of green energy throughout the year and the large amount of MWs required for the clinkerization. For a combustion process, 100 MW for instance is easily and inexpensively achievable, while for a green electrification, from solar energy for example, would require a few square kilometers worth of solar panels, a electrical sub-station and all the maintenance associated with these equipment.

Carbon Capture, Utilization, and Storage (CCUS) is seen as a major technology to help mitigate climate change by reducing greenhouse gas emissions from industrial and power generation sources by 2050. Industrial installations with different technologies are already in place, however, the high costs, energy requirements and necessity of practical experience collection of CCUS remain significant challenges for its widespread deployment for the moment. The intensive investments in this technology tend to revert this situation in the next years.

Apart from those, there are several other technologies, still in their infancy, which still require substantial research, investments, scaling up, to allow for their industrial use. It is an extensive topic that requires a complete separate article for proper discussion.

Conclusion

The cement industry faces significant challenges in reducing CO₂ emissions, but numerous pathways exist to achieve substantial reductions, each one with its characteristics, challenges, costs and CO₂ reduction potential.

It is important to mention that the solution toward CO₂ neutrality is a combination of different methods.

Although at the end of the path, all plants would include most of technologies discussed, each plant is currently in a different stage of decarbonization. Each plant’s unique circumstances require tailored solutions. For instance, there would not make much sense for a plant to invest in calcined clay or hydrogen combustion, while the plant efficiency and use of alternative fuels is not maximized.

By improving plant efficiency, utilizing alternative fuels, replacing clinker with SCMs, and exploring hydrogen use, and the new advances and technologies yet to come, the industry can move toward a sustainable future.

Figure 7. Comparison between different technologies for CO₂ emissions reduction. Source: FCT