Before organic waste, sludges, or cakes can be efficiently converted into energy through thermal degradation processes like pyrolysis and gasification, they need to reach the right moisture content. Too much water in the feedstock means wasted energy, inconsistent processing, and reduced system performance. This is where indirect thermal drying comes in — a proven, energy-efficient pretreatment step that has been used for years across landfills, cement factories, and waste management facilities worldwide.
What Is an Indirect Thermal Dryer?
The Organics thermal dryer is an indirect heat exchanger built around a hollow screw conveyor. Instead of applying direct heat or flame to the waste material, the system heats the waste indirectly as individual particles come into contact with the heated surfaces of the rotating screw. This gentle, controlled approach to drying reduces the risk of scorching, contamination, or uneven moisture removal — all common issues with direct-fired drying systems.
The dryer is specifically engineered to prepare solid waste, cakes, and sludges for downstream thermal treatment, most commonly ahead of a pyrolyser or gasifier.
How the Drying Process Works
At the heart of the system is a hollow screw fitted with heated flights. Waste heat recovered from an external source — typically a thermal oxidiser — is redirected into the dryer system and used to heat oil. This oil is then continuously circulated through the hollow screw, maintaining even, consistent temperatures throughout the length of the drying process.
As waste material enters through the feed hopper, the rotating screw pushes it steadily toward the outlet. Each screw flight has both a flat surface, which pushes the waste forward, and a curved surface. The screws sit within a trough that holds the material in place as it travels the length of the dryer, while motors and a bearing system drive the rotation.
As the waste progresses through the screw conveyor, moisture evaporates gradually until the desired final moisture content is reached.
For design purposes, waste typically enters the dryer at up to 40% moisture content and is reduced to 20–25% before moving on to the pyrolyser.
Temperature Profile
To achieve a 20% moisture reduction, the system is generally designed around the following temperature targets:
- Waste entering the dryer: expected to reach approximately 100°C
- Waste exiting into the pyrolyser: expected to cool to approximately 35°C
The Organics engineering team works with each client to advise on the correct dryer configuration based on specific waste composition and incoming moisture levels.
Why Indirect Heat Drying Makes Sense
This drying technology isn’t new — it’s a well-established method already implemented at landfill sites, cement factories, and various waste management facilities globally. Its continued use comes down to several practical advantages:
- Versatile design that adapts to a wide range of waste streams
- Limited wear and tear thanks to efficient product conveyance
- Wide operating temperature range for flexibility across applications
- Excellent product temperature control throughout the process
- Low power consumption relative to throughput
- Various drive arrangements to suit individual site requirements
- Elimination of product contamination, since heating is indirect rather than direct
Energy demand is low: approximately 8 kWth is required to process each tonne per hour of waste throughput.
Pre-Treatment and Feedstock Requirements
The dryer inlet is designed to handle waste particles of 50 mm or smaller. Where incoming material is larger or more variable in size, a shredder can be fitted at the front end of the dryer to bring particle size within range before drying begins.
Technical Specifications
Each dryer unit is available in three standard throughput configurations, and multiple units can be combined to meet larger system requirements.
| Unit Capacity | Number of Screws | Screw Length | Screw Diameter | Motor Size |
|---|---|---|---|---|
| 1 tph | 2 screws | 7.5 m | 600 mm | 7.5 kW |
| 2 tph | 4 screws | 7.5 m | 600 mm | 15.0 kW |
| 4 tph | 4 screws | 8.5 m | 900 mm | 30.0 kW |
Additional Specifications
| Parameter | Detail |
|---|---|
| Construction material | 304-grade stainless steel |
| Heat source | Redirected exhaust gas from a thermal oxidiser |
| Energy requirement | ~8 kWth per tonne per hour of throughput |
| Max inlet particle size | 50 mm (shredder available if needed) |
| Land footprint | Small |
| Throughput range | 1–4 tph per unit (combinable for larger systems) |
Typical Applications
This drying technology is well suited to facilities managing:
- Municipal solid waste destined for pyrolysis or gasification
- Sludge and cake by-products from wastewater or industrial processes
- Landfill diversion projects seeking to convert organics to energy
- Cement factory waste-derived fuel preparation lines
Key Takeaways
- The Organics thermal dryer uses an indirect, hollow-screw heat exchanger design to safely and evenly dry organic waste, sludges, and cakes.
- Waste heat recovered from an external thermal oxidiser powers the system via a closed oil-heating loop — no additional primary fuel source is required.
- Moisture content is typically reduced from around 40% down to 20–25%, preparing feedstock for efficient pyrolysis or gasification.
- Units range from 1 tph to 4 tph, built from 304 stainless steel, and can be combined for larger-scale operations.
- Energy consumption is low, at approximately 8 kWth per tonne per hour processed.
Get in Touch
If you’re evaluating pretreatment options for a pyrolysis, gasification, or waste-to-energy project, our engineering team can help assess your waste stream and recommend the right dryer configuration for your throughput needs. Contact us today to discuss your project requirements.