Drying of alternative fuels and refuse derived fuels

The question what types of alternative fuels are currently used in the cement industry must be clarified. Various types of alternative fuels are currently being used to replace traditional fossil fuels such as coal, natural gas and oil. These alternative fuels come from different sources and can be divided into different categories:

Biomass fuels (such as wood waste, agricultural waste, as well as biogenic waste from the food industry);

Secondary fuels (such as old and used tires, non-recyclable plastics, industrial waste);

Municipal waste (household waste, processed waste; municipal sewage sludge); 

Industrial by-products (sewage sludge, oil sludge from oil processing and refineries)

and other fuels (“meat&bone”, solvents, etc.)

Furthermore, the ecological advantages of using alternative fuels in cement production must be highlighted.

Substitute fuels bring about a substitution of fossil fuels and thus allow the limited fossil sources to be conserved. Substitute fuels sometimes contain significant proportions of biomass and other CO₂-neutral fuels. As the amount of CO₂ released during combustion corresponds to the amount absorbed by the plants during their growth, they are considered climate-relevant and CO₂-neutral. This significantly reduces the relevant carbon dioxide emissions.

Many alternative fuels come from waste that would otherwise be disposed of in landfills. By using them in cement production, the volume of landfill is reduced, resulting in a lower environmental impact and a longer lifespan for landfills. By substituting fossil fuels, the limited fossil resources are replaced and contribute to the conservation of resources.

Alternative fuels, particularly those made from organic waste, can release lower levels of sulphur, nitrogen oxides and other pollutants than fossil fuels, which can help to reduce emissions and improve air quality. Using waste as fuel supports the concept of the circular economy by reusing and recycling materials rather than disposing of them. This promotes the sustainable use of resources and minimizes waste.

What challenges and risks are associated with the switch to alternative fuels in the cement industry?

The switch to alternative fuels in the cement industry brings with it various challenges and risks, which can be of a technical, economic and ecological nature. Existing cement plants in particular may need to be upgraded with suitable storage, dosing and transportation equipment in order to be able to use alternative fuels efficiently. The combustion properties of alternative fuels differ from those of fossil fuels. This may require adjustments to combustion technologies and processes.

The quality of alternative fuels (AF) can vary greatly, which can affect the consistency and stability of cement production, and drying of AF plays a key role in this.

Some alternative fuels can lead to higher emissions of certain pollutants, which may require additional emission control measures. The combustion of alternative fuels results in the introduction of fuel ash or other residues into the kiln process. These ash components must be taken into account in the operating mode and chemical control of the clinker process.

In addition, the prices and qualities of alternative fuels can be volatile, particularly if they are in demand from other industries or are subject to seasonal fluctuations. Competition for certain waste materials or biomass can lead to bottlenecks and price increases.

To ensure the desired cement quality, adjustments must be made to the production process and the fuels and production parameters must be carefully monitored and controlled. The drying of AF plays a central role here.

Alternative fuels are more cost-effective than traditional fossil fuels such as coal or natural gas. Waste materials that would otherwise have to be disposed of are often made available at a lower cost, free of charge or even at an additional cost, depending on demand and quality.

Although substitute fuels originate from international markets, as they are available locally after their original material use and after processing, they are ideally not subject to high dependencies on the international fuel market.

The conversion of existing cement plants to alternative fuels requires investments in flexible and modular plant engineering or technologies. Maintenance and operating costs must also be considered, as the plant technology is more complex and requires more frequent maintenance.

The implementation of alternative fuels in cement production has resulted in different experiences worldwide. Some countries and companies have achieved remarkable successes and gained a lot of experience in successful use.

Germany is a leader in the use of alternative fuels in the cement industry

Many cement plants use a high proportion of refuse derived fuels (RDF) such as used tires, plastic waste and biomass. This switch has led to significant reductions in CO₂ emissions and dependence on fossil fuels. The proportion of alternative fuels in Germany is over 70% on average [Verein Deutscher Zementwerke e.V., Hrsg. Zementindustrie im Überblick 2023/2024; Stand Oktober 2023. Berlin, 2023, page 19]. Successful plants also achieve annual average substitute fuel rates of well over 95%.

The use of substitute fuels must always be considered with the aim of reducing climate-relevant, political and corporate targets. In particular, the cement industry is predestined to play a significant role in social tasks due to the immense possibilities of conserving resources while at the same time implementing the circular economy concept and reducing CO₂ emissions.

These possibilities with parallel optimization of fuel costs and reduction of international dependencies make the use of substitute fuels (in developed markets) the logical conclusion. The adjustments to be made in terms of both plant technology and operating methods are tasks that should not be underestimated, but which can be successfully and profitably mastered. The successes of the German cement industry, in particular, are emphatic proof of this. 

An essential step towards the successful use of substitute fuels lies in the availability and uniformity of the materials used. As substitute fuels often have very fluctuating properties, the technology used must be able to react flexibly to these fluctuations. One parameter that is often subject to major fluctuations is the material moisture content of the delivered fuels. As the moisture has a direct influence on thermodynamic processes and the calorific value, it has a direct impact on the operation of the stove. It is therefore very important that the material moisture is as low and uniform as possible.

However, it is often not possible to actively influence the available material qualities as they are supplied ready-made by the upstream supplier. One very successful way of exerting influence is to dry the material directly in the factory. The cement process, with its many possibilities for using existing waste heat, is perfect for this. The quality of the available fuel can be considerably improved without the need for additional primary energy, with the aim of significantly increasing the overall substitute fuel rate. Depending on the framework conditions, even previously unusable fuel streams can be processed and made usable for the cement process.

The successful implementation of suitable drying systems in an existing plant requires a variety of interfaces to the existing processes. In addition to the dimensioning of the system, the available waste heat potential and existing restrictions are also evaluated and the exact system setup, including material supply and removal, is defined.

One example of the successful implementation of such drying systems is Broceni Schwenk Cement, which makes it possible to reduce material moisture to an optimum level by using waste heat. Following stelas’ technology supports maximum efficiency and sustainability in the production process of the cement plant.

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