Alternative fuels that are too wet are often not accepted by cement plants and power stations – or lead to process problems: The flame or combustion temperature drops, controllability suffers, and clinker quality in the cement kiln can be impaired. A typical upper moisture limit for delivery is 20 to 25%; many buyers require significantly less. Drying solves this problem directly: It increases the calorific value, makes the material process-reliably meterable, and is the prerequisite for the high thermal substitution rates of 60 to over 80% that leading European cement plants achieve today.

All three are solid alternative fuels processed from waste, but they differ in their degree of processing and quality assurance. RDF (Refuse Derived Fuel) refers to processed municipal waste with a defined calorific value. SRF (Solid Recovered Fuel) is a standardized, quality-assured secondary fuel according to EN 15359 – the standard requires, among other things, the declaration of calorific value, chlorine, and mercury content, which qualifies SRF for use directly at the main burner. SSW (Solid Shredded Waste) is coarser shredded material from waste processing. In practice, the specific specification is more decisive for the drying requirements than the designation – STELA designs the system based on the actual material properties.

The requirements vary depending on the plant, kiln or boiler type, and national regulations. As a guideline: Many cement plants only accept solid alternative fuels up to a moisture content of about 30% – for SRF according to EN 15359, a moisture content of less than 5% is often required for use directly at the main burner. SSW with input moisture levels of 30 to 45% must generally be thermally dried before use. STELA designs the system for the specifically agreed target values and ensures compliance through automatic final moisture control. 

Drying improves economic efficiency on several levels simultaneously: The calorific value increases, which enables higher revenues or avoids surcharges for material that is too wet. The lower weight due to water removal directly reduces transport costs – less water means less tonnage. In addition, storage stability improves significantly, as biological activity and odor development are reduced. For the cement plant or power station itself, the in-house drying system usually pays for itself through saved primary fuel costs and the increased substitution rate, as well as the tradability of freed-up CO₂ certificates.

Cement plants typically have several usable waste heat sources: cooling air from the clinker cooler, heat from ORC processes, or from combined heat and power (CHP) generation. When using clinker cooler exhaust air, the approximately 250 °C process air is cooled down to the drying temperature of about 100 °C with fresh air and used directly for drying. STELA belt dryers can be operated with thermal oil, saturated steam, or already heated process air. The result: The drying system operates with "free" energy that would otherwise escape unused – operating costs remain correspondingly low.

Drum dryers operate with process temperatures of over 120 °C. This has several disadvantages for alternative fuels: plastic-containing fractions can decompose or become sticky, volatile organic compounds (VOCs) are released to a greater extent and increase the effort required for exhaust air purification, and the risk of fire and explosion increases. STELA belt dryers, on the other hand, operate in a temperature range of 60 to 120 °C, dry the material gently, and can be operated directly with the plant's own waste heat. The air-permeable drying belt also acts as a self-cleaning filter, which generally eliminates the need for a separate exhaust air filter system.

Solid alternative fuels can form dust fractions during the drying process that are explosive under certain conditions. STELA takes this into account through adapted design measures and monitoring systems and – where necessary – through a complete explosion protection concept based on the CE risk assessment. The specific measures required depend on the drying product, dust characteristics, and local regulations and are evaluated together with the customer during the early sales phase.

A blanket statement is not possible without concrete plant data – throughput, input moisture, available waste heat, and the current market price for primary fuel are decisive. As a guideline: When using the plant's own waste heat and a throughput of several tons per hour, payback periods of less than two years are typical. In addition, there are factors that are difficult to quantify, such as avoided surcharges for non-specification material and improved planning reliability for the combustion process. STELA prepares an individual profitability calculation as part of the project consultation. 

STELA belt dryers are designed for all common solid alternative fuels: RDF (Refuse Derived Fuel), SRF (Solid Recovered Fuel), SSW (Solid Shredded Waste), fluff, as well as secondary fuels (BGBS) and other high-calorific fractions. Since these materials can vary greatly in composition, moisture, and flow behavior from batch to batch and depending on the season, an automatic final moisture control is a standard component at STELA. In our in-house technical center, drying tests can be carried out in advance to design the system exactly for the respective material.

Yes. STELA drying systems are delivered as turnkey units and can be integrated into existing plant layouts – even in confined spaces. Connection to existing conveyor technology, heat supply, and control systems is part of the standard scope of project planning. STELA accompanies the customer from the initial design calculation to commissioning.

Yes – low-temperature drying shows its strengths especially in greenfield projects. Because the systems can use waste heat from upstream or parallel process stages, the energy demand can be integrated into the overall concept from the beginning – this lowers operating costs and emissions over the entire service life. The modular, scalable design also allows the drying process to be precisely tailored to the planned capacity and to grow as needed. In combination with gentle material handling and low regulatory hurdles, low-temperature drying is thus a future-proof basis for new plant concepts. STELA accompanies such projects from the initial design calculation to commissioning.

The BT (belt widths of 6,200 and 8,400 mm) is the high-performance dryer for cement plants and power plants with high fuel demand. The BTL (belt widths of 2,000 and 3,000 mm) is suitable for small to medium capacities or for entering the field of alternative fuel drying. Both types operate according to the same process engineering principle and are individually configurable.

STELA belt dryers are designed for continuous 24/7 operation and typically run for 2,000 to 2,500 operating hours without interruption. The drying belt itself has – with over 25 years of practical experience – a service life of up to 40,000 operating hours and can be replaced by the operator's personnel if necessary – typically in 4 to 6 hours with a two-person team. A product change is possible without emptying the dryer. Experience shows that annual maintenance costs are around 1% of the original purchase price. Especially for operators in continuous shift operation, this low maintenance requirement is an important factor for overall economic efficiency.