Biomass pre-drying for renewable fuels – reliably solved by STELA

Freshly delivered waste wood or other biogenic material typically has moisture contents of 40% to over 60%. However, pyrolysis, gasification, and HTL plants require a narrowly defined, uniform input moisture to operate stably and with maximum yield: Material that is too wet lowers the calorific value, destabilizes process control, and reduces the fuel yield per ton of biomass used. Drying is therefore not an optional preliminary stage, but a process-technical necessity for almost every conversion method.

The requirements differ significantly depending on the process. For fast pyrolysis, a residual moisture of less than 10% is considered a guideline for achieving stable pyrolysis oil yields. For gasification and hydrothermal liquefaction (HTL), the target values vary more depending on the plant manufacturer and process control – for HTL, for example, a more moderate pre-drying may be sufficient, as the process itself works with water as a reaction medium. STELA designs the drying plant exactly to the target value agreed upon with the plant manufacturer.

All three processes convert solid biomass into higher-energy intermediate products, but differ fundamentally in process conditions and products. Pyrolysis heats biomass in the absence of oxygen (in fast pyrolysis to around 450–600 °C) and primarily produces pyrolysis oil as well as biochar and gas as by-products. Gasification operates at higher temperatures with a limited amount of oxygen or steam and produces synthesis gas (CO and H₂) as a basis for Fischer-Tropsch fuels, methanol, or synthetic natural gas. Hydrothermal liquefaction (HTL), on the other hand, uses water as a reaction medium under high pressure and moderate temperatures and converts biomass directly into bio-oil. For drying, this difference is crucial: pyrolysis and gasification require a dry, narrowly defined input moisture (under 10% for fast pyrolysis), while for HTL – which works with water anyway – a more moderate pre-drying is often sufficient. In any case, STELA designs the drying plant to the target value agreed upon with the plant manufacturer.

Dried biomass has a higher calorific value and provides a higher, more stable yield of bio-oil, synthesis gas, or other intermediate products. The lower weight due to water removal also reduces transport costs. If the drying plant is operated – as in the Arbios Biotech practical example – with excess process heat from the conversion process itself, the operating costs of drying are additionally reduced significantly, as no additional primary energy is required.

Yes. A key advantage in the field of renewable fuels is that conversion plants often generate waste heat themselves, which can be used directly for upstream drying – as in the Arbios Biotech project, where the belt dryer uses a large part of the excess process heat from the HTL process. Furthermore, STELA belt dryers are compatible with thermal oil, saturated steam, or hot water from combined heat and power (CHP) and do not require waste heat to function – they use it, where available, to reduce costs.

STELA belt dryers process the entire spectrum of biogenic feedstocks: forest residues, sawmill residues such as bark and wood chips, landscape maintenance material, as well as agricultural residues such as straw and harvest residues. Since these materials can vary greatly in moisture, particle size, and composition depending on origin and season, an automatic final moisture control is a standard component at STELA. In the in-house technical center, drying tests can be carried out in advance to design the plant exactly for the respective material.

That depends on the respective project. In the Arbios Biotech project, the STELA belt dryer processed sawmill residues with a particle size of under 25 mm at a throughput of up to 80 m³/h. Other particle sizes and material mixtures are possible, but require project-specific adjustments to belt configuration and feeding technology – STELA provides advice on this during project planning.

Drum dryers operate with process temperatures well above 120 °C. This can entail several disadvantages for biogenic feedstocks: increased release of volatile organic compounds (VOCs), higher fire and explosion risk, and – depending on the conversion process – undesirable thermal pre-reactions in the starting material. STELA belt dryers operate in a temperature range of 60 to 120 °C, dry the material gently – the product itself typically leaves the dryer at under 40 °C – and can be operated directly with existing waste heat from the conversion process, which drum dryers often cannot do in practice. The air-permeable drying bed also acts as a self-cleaning filter, which generally eliminates the need for a separate exhaust air filter system.

The ReFuelEU Aviation Regulation mandates a gradually increasing blending mandate for Sustainable Aviation Fuel (SAF) for airlines in the EU: 2% from 2025, 6% from 2030, 20% from 2035, and 70% from 2050. A growing portion of this demand must be met via biomass-based processes such as pyrolysis, gasification, or HTL, as well as via synthetic fuels, since the use of used cooking oils (UCO) is limited by the RED III cap. This also increases the demand for efficiently pre-treated, biogenic raw material bases in the medium term – and for the drying technology required for this.

Biochar is the solid, carbon-rich by-product that is created during the pyrolysis of biomass. It is gaining economic importance because it opens up several revenue streams: as a soil improver in agriculture, as a long-term CO₂ sink in the context of carbon removal certificates, and as a reducing agent or carbon carrier in industrial processes. A prerequisite for stable biochar quality—as it is for high pyrolysis oil yields—is a defined, low input moisture content of the biomass. This is precisely where pre-drying with a STELA belt dryer comes in: it brings the starting material uniformly to the moisture range required by the pyrolysis process. Whether and to what extent STELA can also handle the post-drying or conditioning of the finished biochar must be clarified on a project-specific basis.

Biomass dusts can form explosive dust-air mixtures during the drying process. STELA takes this into account through adapted design measures, dust extraction, 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 project data—the decisive factors are throughput, input moisture, available waste heat from the conversion process, and the market value of the fuel or intermediate product produced. If the drying plant can be operated with excess process heat—as in the Arbios Biotech project—the additional energy requirement drops significantly, which has a positive effect on the payback period. STELA prepares an individual profitability calculation as part of the project consultation.

The BT (belt widths of 6,200 and 8,400 mm) is the high-performance dryer for plants with large biomass throughput, such as at integrated biorefinery sites—a BT 1/6200-12, for example, is in use at Arbios Biotech. The BTL (belt widths of 2,000 and 3,000 mm) is suitable for pilot plants and projects with lower or variable throughput. Both types operate on the same process engineering principle and are individually configurable; if required, RecuDry® heat recovery can also be integrated.

Yes. STELA drying plants are supplied as turnkey units and can be integrated into both existing plant layouts and newly planned greenfield projects. Especially with novel conversion processes like HTL, whose process parameters are often developed on a project-specific basis, STELA works closely with the plant manufacturer to design the drying plant to precisely match the downstream process—from the initial design calculation to commissioning.

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—with over 25 years of practical experience—has 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, which minimizes downtime, especially with changing feedstocks. Experience shows that annual maintenance costs are around 1% of the original purchase price. For applications in the renewable fuels sector—usually in continuous shift operation—this low maintenance requirement is an important factor for the overall economic efficiency of the conversion plant.