Design and Construction of the Rannu Boiler Plant

The fully automated boiler plant with a woodchip storage operates on wood-based waste fuels (bark, sawdust, etc.) with a moisture content of 30–60% and a particle size of up to 100 mm. The nominal capacity of the boiler is 0.85 MW (at 50% moisture), and the automatic storage volume is 140 m³. An emergency generator is provided to protect the boiler in the event of overheating or power outages. The boiler is equipped with all necessary control, safety, and monitoring devices. A mixing pump is installed to maintain the temperature of the return water to the boiler. Pipeline valves and fittings comply with pressure class PN16. To measure boiler output and produced heat, a Kamstrup Multical heat meter with an ULTRAFLOW flow sensor is installed.

Fuel loading into the woodchip storage takes place directly from the vehicle. Fuel is transported from the storage to the boiler feed point by screw conveyors, and the final feeding into the boiler is performed by an auger.
The fuel feed line is equipped with an automatic fire extinguishing system.

Technical Description of the Hot Water Boiler

The boiler is designed and tested according to PED 97/23/EC, Article 3, Section 3. The operating temperature is 110 °C and the working pressure is 6 bar. It is a 5-pass boiler with an open bottom. The combustion chamber is designed using the latest technologies to maintain a minimal flue gas temperature, ensuring maximum heat extraction. The boiler is welded from certified steel, insulated with 100 mm mineral wool, and covered with sheet metal panels. For the automatic cleaning of the flue gas tubes from fly ash, the DANBLAST compressed-air system is used.

1. Combustion Chamber

The chamber is water-cooled and equipped with a moving grate. The stepped moving grate includes water-cooled pipes that maintain a constant grate temperature. This cooling technology prevents slag buildup and significantly extends the service life of the cast-iron grate elements. The chamber walls are also water-cooled.

2. Moving Grate

The grate consists of interconnected moving steps to ensure a smooth combustion process. Movement is driven by a hydraulic cylinder connected to a hydraulic power unit. The movement speed is adjustable to ensure complete combustion.

3. Primary Air

Primary air enters through openings in the grate, while secondary air is supplied through pipes located in the side walls. Primary and secondary air flows are regulated separately to achieve optimal combustion.

4. Dry Ash Removal

Ash from the boiler and from under the multicyclone is transported into a 10 m³ ash container.

5. Multicyclone

The multicyclone contains several simple cyclones. Due to the high gas velocity and cyclone efficiency, solid particles separate and settle at the bottom. The outside of the unit is insulated with 100 mm mineral wool and covered with sheet metal.

6. Induced Draught Fan

The fan ensures proper negative pressure in the combustion chamber, correct gas flow through the multicyclone, and stable draught in the chimney. It is equipped with a vibration damper.

7. Control Automation

The control system monitors and adjusts all parameters within a power range of 30–100%. The control panel displays the boiler’s current useful output and other operating data. The system is equipped with a touchscreen display, a PLC controller, frequency converters for all fans, a frequency converter for the induced draught fan, and dedicated control software.
An internet-based monitoring system allows remote supervision and adjustment of the boiler unit.

8. Fuel Storage

The active storage volume is 140 m³, filled directly from the vehicle. Fuel feeding is handled by hydraulic scrapers, a hydraulic power unit, and a screw conveyor. The conveyor includes an automatic fire extinguishing system that prevents fire from spreading into the fuel storage. The system operates independently of external electrical power.