Increasing AF usage at the kiln

Cement plants are striving to reduce fossil fuel usage for cost savings and to lower CO₂ emissions by adopting alternative fuels (AF) like solid recovered fuel (SRF) for the kiln burner and refuse derived fuel (RDF) for the calciner burner. The goal is to achieve a 100% thermal substitution rate (TSR) without compromising clinker quality, the pyro process, or environmental impact.

However, various limiting factors must be addressed. Since many alternative fuels have high moisture content, drying them before use is essential. Additionally, particle size and shape significantly influence combustion. While coarser 3D particles can be used in the calciner, the kiln burner or satellite burner requires fine and flaky 2D materials. The use of a satellite burner further enhances combustion and flame stability in the rotary kiln. This article emphasises the importance of drying and classifying alternative fuels and their impact on clinker and cement production.

Importance of drying alternative fuels

The drying of alternative fuels plays a significant role in the clinker burning process in the cement industry. Alternative fuels are increasingly being used as a sustainable substitute for fossil fuels to reduce CO₂ emissions.

Since many alternative fuels have a high moisture content, it is important to reduce the water input by drying them before co-processing. High moisture content cools the flame at the kiln burner and reduces thermal NOx emissions, but it also reduces kiln performance and dramatically increases ID fan power consumption by raising exhaust gas volume.

While fossil fuels like coal dust can be assumed to be uniform in terms of size, density, and shape, the situation becomes different when using AFs. The particles of waste-derived SRF consist of different densities and shapes, resulting in different chemical as well as physical properties when leaving the tip of the kiln burner. The risk of affecting the pyro-process and clinker quality increases with the level of AF substitution rate.

Insufficiently pre-processed alternative fuels are characterised by a high variation in moisture content. During winter, the moisture content can often exceed 20% due to the management of input materials. The fluctuating moisture content of SRF has a significant impact on stable kiln operation as it also affects the calorific value. Additionally, the moisture cools the flame of the kiln burner, leading to poor burning conditions and weakly burned clinker with a high free lime content. Furthermore, the presence of compact 3D particles, an inhomogeneous composition, and insufficiently extracted ash components such as metals, glass, ceramics, and stones will affect the clinker quality by altering its ash chemistry. The high content of volatile circulating elements such as chlorine, sulfur, and alkalis also negatively impacts process control.

To ensure adequate fuel quality, especially at the kiln burner, it is necessary to guarantee that the SRF has a low moisture content, a large particle surface free of 3D particles, a suitable and consistent ash composition, minimal impurities, and a low content of cycle-forming elements.

Several technologies are available for drying alternative fuels, including drum drying, belt drying, and flash drying.

• In drum drying, the alternative fuels fly through a rotating drum while hot air circulates, carrying only the light, dried particles. The heavy, moist particles remain until they can be captured by the hot gas and discharged in a cyclone.
• In belt drying, the alternative fuels are dried while lying on a slowly running belt. The air is heated indirectly via radiators from above or below. In belt dryers, hot air can also be efficiently directed through the conveyor belt to dry the material. At the end of the belt, all particles are discharged, regardless of whether they are completely dry or not.
• Flash dryers use the turbulence of hot exhaust gas mixed with AFs to dry them in suspension. Heavy particles are separated in the process, but without any control. Drying the alternative fuel in a flash dryer significantly reduces its moisture content.

Depending on the moisture content of the input material, some plants have reported a residual moisture of 5 – 9% after the flash dryer. For higher moisture content, cement plants also use belt dryers. Usually, hot exhaust air from the clinker cooler serves as the heating and drying medium. By taking the dryer exhaust air to the first compartment of the clinker cooler, drying occurs in a closed circuit, avoiding any adverse effects on the environment.

By adjusting the air flow (lifting speed) in a flash dryer, reliable removal of impurities is possible. Moreover, flash dryers offer lower investment costs compared to other systems.

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Read the article online at: https://www.worldcement.com/special-reports/22122023/increasing-af-usage-at-the-kiln/