As more and more emissions are released and warms the Earth’s atmosphere, global non-government organisations (NGOs) and policymakers are uniting to limit the release of these unwanted greenhouse gases. In response, cement plant operators are seeking innovative ways to maintain healthy production levels and simultaneously reduce their CO2, NOX and other emissions. This often requires in-depth knowledge of production processes, as well as adjusting existing cement manufacturing lines and cement composition.
The complexity of the cement production process from start to finish is a labyrinth of scorching temperatures, numerous chemical reactions, lightning-fast heating & cooling and at times hard-working catalysts.
It is critical to understand the entire process because making even the slightest change to one part of the process can start a chain-reaction.
The heat is on
In cement manufacturing, clinker production demands the most energy and emits the greatest amount of CO2 into the atmosphere. Clinker is produced at a sizzling 1,450 °C (2,640 °F), a crucial point for certain chemical reactions to occur.
With such a high energy intensity, it makes sense to focus optimisation around clinker production to lower overall energy consumption and reduce CO2 emissions. Of course, it is not the only place where improvements can be made. Decreasing the clinker to cement ratio by substituting clinker with other materials is just one proven way of doing so. This practice is aptly named clinker substitution.
Passing the strength test
Using clinker substitution to reduce CO2 emissions is increasingly being considered by many cement plant owners. Typical cement composition is 95% clinker, with gypsum making up the remaining amount. By substituting clinker with a comparable material, cement plant owners can achieve impressive environmental gains and align with global efforts to reduce CO2 emissions.
For a material to be deemed suitable for clinker substitution, its compressive strength, tensile strength and water absorption are must-have characteristics. With cement being used to construct buildings and bridges, its composition needs to be able to withstand, for example, the devastating effects of an earthquake.
Strict cement strength standards affect the decision to adopt the practice of clinker substitution. For instance, the European cement standard, EN 197-1, outlines methods of testing cement to determine its strength. These standards, and others like it, may be viewed as roadblocks for considering clinker substitution. It isn’t as difficult as it seems, though, especially with a team of cement processing experts ready and willing to help.
Supplementary cementitious materials
Using supplementary cementitious materials (SCMs) is an effective way to reduce the clinker factor in cement production. It decreases cement production costs and reduces energy consumption without compromising cement quality.
In addition to reducing the clinker factor, SCMs contribute to the strength properties of cement through hydraulic or pozzolanic activity.
Just as strong
So, what is considered a good substitute? Many cement plants around the globe making sustainability-oriented upgrades are turning to clay calcination. Some recent examples are shown in South America and the Philippines, where clay is readily available and suitable for clinker substitution. Certain parts of Brazil are also abundant in clay that has the stamp of approval for calcination.
As with all things novel, it’s fair to be sceptical. And with such strict standards outlining required strength and other characteristics, why change something that has worked well since cement production began?
It’s about balancing the profitability of the cement plant with taking ownership of the environmental footprint of the business. The effects of climate change are felt across the world – fast becoming a reality in many people’s everyday lives. A readily available solution that the industry can use today is clay calcination as a tried and tested method.
Clay can make your day
For many cement plant owners, some points need to be considered before committing to clay calcination for clinker substitution. Firstly, it is ideal that the cement plant is located close to a clay quarry. Transporting large quantities of clay over long distances defeats the overall purpose of lowering CO2 emissions.
Secondly, a heat source is required for clay calcination. Temperatures of around 700-800°C will suffice, and it is feasible to reuse old equipment like rotary kilns. However, it’s not a simple feat. The heat balance needs to be just right throughout the entire process to produce gas hot enough for calcination. Skaarup’s team of technicians test clay samples at their labs in Denmark and India. She says: “Once the clay has been activated, the lab team can test it for strength and other desirable characteristics. It needs to meet the applicable standards before we pass it.”
Another consideration is that clay naturally has a red-brown hue, which would mean the environmentally friendly cement will not be the typical grey or white colour. Of course, the colour does not impact the strength of calcined clay.
Swiss spotlights sustainability
An intensive research project called the LC3 project is currently being undertaken by the Swiss government. The project finances sustainability projects within the cement industry. One of the projects explores the composition of limestone and clay, as well as the potential of widespread adoption of Limestone Calcined Clay Cement (LC3). A typical flash calciner can be used to make LC3. Skaarup comments: “I think that within two years, the EU standards will adapt to fit the widespread use of LC3 cement. India’s standards are expected to change this year. Hopefully, this will be the beginning of change in the industry where clinker substitution becomes the norm.”
The bigger picture
While clinker substitution is one way of helping to reduce emissions, there are certainly other parts of the process that can be targeted. For example, the grinding process, which requires considerable amounts of electrical energy. The widespread adoption of energy-efficient technologies, such as vertical roller mills and hydraulic roller presses, in place of traditional ball mills should be encouraged.
There needs to be a greater push for cement plants to always be using more energy-efficient equipment.
What about alternative fuels?
With sustainability high on the agenda, alternative fuels firing is fast becoming more common in cement plants. It complements clinker substitution as a solution to reduce emissions. Here are some facts about one type of alternative fuel: refuse derived fuel (RDF):
- RDF is composed of common household waste
- For lab technicians, RDF is difficult to analyse as it lacks consistency in composition
- The unpredictable burn quality of RDF is why it is mainly used in the preheater
- The moisture in RDF needs to be evaporated for good combustion to occur, resulting in good burn quality